>>Check the rules
Parks release spots on a regular schedule as far in advance as six months. But some parks accept applications before the window opens and pool them in a lottery drawn the first day that permits can be reserved. Check individual park websites for rules. (Morally dubious but unquestionably effective: Have members of your group submit multiple permit applications.)
No reservations? Don't give up. About 40 percent of all spots are reserved for walk-up permits that are released 24 to 48 hours before the trip's start date.
Some parks (Yosemite, for instance) manage hiker numbers by imposing trailhead quotas instead of campsite restrictions. Simply choose a trailhead that's an extra day's walk from the most coveted scenery.