Because so many travelers descend upon national parks in the summer, lodging reservations are essential. This is especially true if you want to stay inside the park, but it also applies to hotels, motels, even teepees outside the parks.

Here are some tips for getting the accommodations of your choice -- and avoiding a night in your rental car. (Note: Lodging mentioned below was available at press time).

* Book early. Many park lodges, particularly the grande dames in the chart above, can sell out months before the start of peak season. Check with the parks (links available at www.nps.gov) to see how far in advance you can book, then reserve when the rooms are up for grabs. (Beware of commercial booking sites that charge fees for making reservations.) The good news: Experts say booking patterns have changed, with many travelers able to book a weekend park stay with a lead time of a month or so, weekdays about two weeks before arrival.

* Take advantage of last-minute cancellations. Tour operators book rooms far in advance. If a tour doesn't fill, they must release the spare rooms. Monitor park Web sites for these last-minute spots. Bold travelers who show up at a lodge looking for a room that night -- or perhaps those with a confirmed motel room seeking something better -- could call the lodge and ask what time no-show rooms are released (usually 4 or 6 p.m.). Then call or visit the lodge the day you want to stay over and see if a room is available. This is a greater gamble at a big-name park like Yellowstone than at Shenandoah, which is not as packed with tourists.

* Pitch a tent. For under $20, you can have a slab of land on which to camp. Many national park campsites are first-come, first-served. Go to the site early to reserve your spot and have cash in hand; most don't accept plastic. Check the park's Web site or the National Park Reservation Service (800-365-2267, http://reservations.nps.gov), which books spots at more than 30 parks, or the National Recreation Reservation Service (877-444-6777, www.reserveamerica.com), which handles campsites and cabins at federal and state parks.

* Go on a package tour that includes lodging. Stay in air-conditioned dorms during a late-August eco-trip to Everglades National Park in Florida through Sierra Club Outings (415-977-5522, www.sierraclub.org/outings; $695). Sleep in lodges in Bryce, Zion and the Grand Canyon on a seven-night trip in July or September with National Geographic Expeditions (888-966-8687, www.nationalgeographic.com/ngexpeditions; $2,595).

* Avoid weekends. Rooms, cabins and campsites -- both in and outside the parks -- book quicker during weekends. The Days Inn (888-425-3113, www.daysinn.com), five miles from Capitol Reef National Park in Utah, is nearly filled for summer weekends but has plenty of $60 to $80 rooms available for weekdays.

* Check the calendar. As soon as you have your vacation date/park in mind, call the area's tourism office to see if there are any major events around the time of your visit. For instance, Aug. 9-14 may be a bad time to visit Mount Rushmore in South Dakota, when motorcyclists come for the Sturgis Rally.

* Book a room, even if your plans aren't definite. This is important at remote parks. Jerry Hyatt, manager of the Ramada Limited Cherokee (800-359-5672, www.cherokeeramada.com; from $99) in Cherokee, N.C., and two miles from Great Smoky Mountains National Park, has allowed tourists to sleep in their cars in the hotel's parking lot, because all area hotels were booked. You can always cancel your room, with proper notice, if you don't go.

* Visit during the summer's "shoulder seasons." June and late August have fewer visitors, thus lodging is easier to come by. Hotels also offer discounts to fill vacancies off-season (generally from September until mid-spring). Check out the Travel Industry Association of America's SeeAmerica.org (www.seeamerica.org) for discounts, such as 20 percent off rooms at the Saddle West Hotel, Casino and RV Resort (800-433-3987) near Death Valley in California.

* Look for interesting options. Just because you're visiting a national park doesn't mean you have to stay in a cabin, hotel or lodge. Check area tourism bureaus for ideas. What about a teepee on a cattle ranch? The Grand Canyon West Ranch (800-359-8727, www.grandcanyonranch.com), along the western rim in Meadview, Ariz., has those for $99 a night (plus meals). B&Bs, such as the Penury Hall B&B (207-244-7102, www.penuryhall.com; from $70), a half-mile from Acadia National Park in Maine, are also popular.

* More info: National Park Service, 202-208-6843, www.nps.gov; National Park Foundation, 202-238-4200, www.nationalparks.org.

-- Elissa Leibowitz

At Yellowstone's Old Faithful Inn, book early to get a room with a geyser view.