How to go with the flow but restricted by reservations?

I usually only make reservations for major holidays or special events. The rest of the time I either get on the phone around 3:00pm or just pull in and ask. Most places have a few sites reserved for overnighters. I can't remember the last time I was turned away because someone was full. If you want to camp inside the national parks, the space is usually limited and so you may need to make a reservation; but for private parks it's usually OK not to.