+1 on 4runnerguy's advice. Exactly how I got a campsite in Bridalveil CG in Yosemite. Once there I stayed 5 nights, and observed that the CG usually didn't fill until sometime between 12:30 pm and 3:30 pm M-Th, but when I left Friday morning there were people waiting to grab a site.
I showed up at Red Canyon CG one June early afternoon and there were several sites left, so I got one and spent the night. Hiking the Arches Trail was a real scenic treat, and the CG had showers which was a plus for me. Nice place.
