In Colorado, State Parks are on reservation systems at http://parks.state.co.us
http://parks.state.co.us/Reservations/Policies/Pages/ReservationPolicies.aspx
Within 3 days of your visit date, remaining sites are first come, first served
National Forest and National Park reservations are made at http://www.reserveamerica.com/ - reservations minimum of 4 days before arrival ... anything not reserved then reverts to first come first served.
Sadly, I've resigned myself to reservations for planned weekends. However, NFS and BLM boondocking opportunities are plentiful in CO.
