Editor's note: Updates with additional details about navigating Xanterra Park & Resort's website.

Are you thinking of spending your summer vacation with a base camp at Old Faithful Inn in Yellowstone National Park, or perhaps planning to view the fall foliage in Shenandoah National Park, Great Smoky Mountains National Park or along the Blue Ridge Parkway with a room in a charming lodge?

Well, it might already be too late to secure the exact accommodations you're seeking for this coming summer or fall.

Rooms are going fast at Old Faithful, with much of the inn sold out for June, July, August, and September, according to Xanterra Parks & Resort's website. Summer openings also are quickly disappearing at Yosemite Lodge at the Falls in the Yosemite Valley. And prime leaf-peeping October weekends in Shenandoah are filling up the rooms at Big Meadows Lodge and Skyland Resort.

So what's a park visitor to do?

In hindsight, planning to stay in one of these highly sought locations should have started as much as a year before your intended stay. But, of course, it's too late now if you're traveling this summer or fall.

Still, some perseverance, and flexibility, perhaps can help. Tour operators typically reserve huge blocks of rooms across the National Park System with hopes of filling their tours. When they don't, they release some of those rooms.

“We tell people 30 days out is a good time to look (for openings), says Xanterra spokesman Tom Mesereau. “Check back often. When people are making their reservations last May for this coming summer, a lot of things can happen in their lives. There are quite a bit of people calling up with cancellations. But again, they can fill up quickly with other people coming in.”

At Forever Resorts, which has properties at such parks as Grand Teton, Bryce Canyon, the North Rim of the Grand Canyon, and Mammoth Cave, Reservations Manager Steve Angell suggests you visit their online reservations pages time and again to watch for openings if your first attempt proves fruitless.

"We've experienced some availability to open up about 30 days in advance all the way down to 72 hours. The availability is sporadic and books quickly for the iconic parks, which is why we recommend customers to check the website for up-to-the-minute availability," he said.

David and Kay Scott, the Traveler's lodging experts and authors of The Complete Guide To the National Park Lodges, recommend that you check the respective property manager's "cancellation policy and check back when cancellations are most likely for the dates you desire."

"For example, if a lodge has a 14-day cancellation policy, check with the lodge for cancellations 14 days prior to the date you would like to arrive. This is when you are most likely to find a vacancy," they say. "Another thing: Consider traveling during the off-season when crowds are lighter. We once stayed at Kettle Falls Hotel in Voyageurs National Park during the last week of the season and we were the only guests in the hotel. "

Above all else, "keep calling," say the Scotts. "Rooms regularly open up as people with reservations cancel. We tried for two days to get a room at Yosemite Lodge in Yosemite Valley before one opened up for us."

Also, remember that rooms without a private bath are often the last to fill, they note. "Bite the bullet and ask about the possibility of one of these rooms, even if you would prefer a room with a private bath," the Scotts suggest.

One other option would be to take a course in a national park such as those offered by the Yellowstone Association Institute or the Rocky Mountain Nature Association. Sometimes these programs include lodging, either in a cabin or lodge or, in the case of backcountry courses, a tent.

Putting Those Suggestions To Use

* Things are going so quickly in Yellowstone that you might soon be left with few options outside of pitching a tent. But before you go into panic mode, dig around Xanterra's website.

When you first plunk in the lodge you want to stay in and the dates you desire, the site's search engine automatically kicks out the availability for the most-desired rooms and a corresponding calendar. So if you first put in your dates without selecting a specific room, the results for a "Hi-Range Room 2 Queens" get kicked out and the associated calendars show the summer being sold out.

But if you use the drop down "room choice" box in the left-hand column you can see what else is available.

Still, even if you go all the way down to a room in the inn's "old house" with two queen beds and the bathroom down the hall you'll see there are relatively few dates to choose from until mid-September.

And by using the drop down box you'll see that while the most popular rooms at Canyon -- Frontier Cabins with two double beds -- are sold out for the summer, there are quite a few other rooms available. And at Roosevelt Lodge, while the most popular cabins, those with two double beds, are all but sold out for the coming summer, there are a good number of cabins with a single double bed yet available.

And there are some other options you can employ to get a room in Yellowstone this summer if you act quickly. For instance, the Grant Village Lodge ($152 per night, double occupancy), just 17 miles from Old Faithful, has great vacancy at this point in time. True, the lodge lacks the ambiance of the stately log inn, but at least you'll be in the park, and Yellowstone Lake is gorgeous.

Another option is to consider the Old Faithful Lodge. This is not an actual lodge with rooms but rather a collection of cabins ($110 night) that are not nearly as nice as a room in the inn but which will place you at Old Faithful just the same. The Old Faithful Snow Lodge also has spotty openings left ($206) for the coming summer months. But don't expect those rooms to last long.

On the northern edge of the park, the Mammoth Hot Springs Hotel also has so-so availability for the most popular rooms, those with two queens and a bath, but there is pretty good availability for rooms with one queen or with a shared bathroom down the hall.

* Sometimes your chances are enhanced if a park has no lodging within its borders because there often are many options just beyond the borders. Acadia National Park is one such animal, as there's a wide range of accommodations outside the park in the Bar Harbor area. For a list of those, visit the Bar Harbor Chamber of Commerce web site. These options run the gamut, from pricey hotels to less-expensive, cozy cabins that have been luring families back year after year after year.

There are alternatives to the Bar Harbor area, of course. The Southwest Harbor and Northeast Harbor areas offer some very nice lodging options within a very short drive of the park. There's the tony Claremont Hotel and Cottages ($200-$335 per night cottages) and the more subdued and affordable Seawall Motel ($80-$115) in Southwest Harbor.

Across the mouth of Somes Sound in Northeast Harbor stand the Asticou Inn with its collection of cottages and inns ($195-$380) and Grey Rock Inn ($110-$375).

* The namesake lodge at Bryce Canyon National Park has a relatively short season in comparison to many other parks, running from April 1 through November 12. That can make things more competitive when it comes to reserving a room.

According to Mr. Angell at Forever Resorts, which operates the Lodge at Bryce Canyon, last year early April and late October tended to be lighter in booking than the heart of the summer, when things generally sell out. As of Sunday, January 23, availability looked good for this summer.

Forever Resorts also operates the Signal Mountain Lodge in Grand Teton National Park. Not surprisingly, in light of its location, this lodge (which operates May 6-October 16) also generally sells out for summer. Unfortunately, you can't check availability or make a reservation on-line; instead, you need to call 307-543-2831 between 8 a.m. and 5 p.m. (Mountain Time) Monday through Friday.

* If you don't mind the heat, summer typically is the slow season at both Death Valley National Park and Big Bend National Park. As a result, you can readily find rooms at the Furnace Creek Ranch ($134-$184/night) in Death Valley or the Chisos Mountains Lodge ($113-$141 per night) in Big Bend.

When A Campground Is All You Seek

Of course, there also are national park campgrounds, a decidedly less-expensive alternative to having four walls, a floor, and ceiling around you. And in many parks you can reserve your summer slice of the outdoors now.

At Acadia, the Seawall (214) and Blackwoods (306 sites) campgrounds offer two woodsy settings that keep you front-and-center in the beauty of the park and just a 10-minute walk from the ocean.

Yellowstone has a dozen options, ranging from the sprawling (425 sites) Bridge Bay Campground to the less crowded Slough Creek (29) sites. Five of the campgrounds (Bridge Bay, Canyon, Fishing Bridge RV Park, Grant Village, and Madison) are managed by Xanterra, and you can check availability and make reservations at their website. The others -- Indian Creek, Lewis Lake, Mammoth, Norris, Pebble Creek, Slough Creek, and Tower Fall -- are run by the Park Service and are first-come, first-served.

Perhaps the easiest way to assess the state of campground fees, availabilities, and reservations across the National Park System is to visit www.recreation.gov . This site makes it easy to check campgrounds in the parks and see what the nightly fees are and where availability can be found.

Considering a Long Hike?

If you're not looking for a room or a campground, but rather a long trail to explore, now also is a good time to be firming your plans up. There are permits to be had, campsites to be reserved, and perhaps even a room to be booked on either end of your trip.

For instance, if you're thinking of a long-distance trek on the John Muir Trail in Yosemite, six months out is not too soon to be checking into permits.

For additional insights into planning long treks along the Appalachian National Scenic Trail, the Pacific Crest Trail, the Continental Divide Trail, or the Wonderland Trail around Mount Rainier, dive into the Traveler's archives and read this planning story from November 2009.

Something else to keep in mind if you're thinking of a shorter backpack trip in a park is that more and more parks are requiring not just permits to enter the backcountry, but campsite reservations. At Yellowstone, for example, you can plot your journey using the park's backcountry planner, fill out a reservation form, and send it in now. Come April 1 the backcountry rangers will take all the applications they've received and do a lottery to determine who gets what sites. After April 1 it's first-come, first-served, so if you have a favorite campsite, you might be wise to nail it down now, even if you're not hiking until September.

Great Smoky Mountains National Park also has a permit system and reservation system for its backcountry. This page provides the insights into how to negotiate those systems.

Another thing to look for are requirements specific to an individual park. For instance, Grand Teton officials require you to have a bear-proof canister to store all "foods, trash, toiletries, and other scented items" for backcountry journeys below 10,000 feet. (If you are thinking of heading to Grand Teton for a backpack trip, they're already taking reservation applications).

To sum it all up, now is the time to be planning and booking your lodgings. And, in many cases, you also can, and should, make dinner reservations at the lodges. Wait much longer and you'll either be eating when you'd rather be out enjoying the park or when you'd prefer to be getting ready for bed.

For more specifics, check not only the website of the park you intend to visit, but also that of the concessionaire.