How likely to get last-minute spots in national parks

Somebody mentioned Lassen as being uncrowded. It is smaller, but the campgrounds are often sold out all summer. Tried to get in for a week in June but was too late in making reservations. Beautiful park, though, once you get in! My advice would be to make reservations where you can and keep checking for cancellations at your first choice park. Check daily. This is how I usually get into Yosemite, when I have missed the on sale date.