A few naughty people make extra reservations and try to 'scalp' their sites. Each day there will be people leaving their camp sites and going home. You can simply search for a better site each day until you get a really awesome one. Most camp grounds do not reserve specific sites - just a site in general. If you arrive at the campground late in the day, you will get your choice of whatever is left - and that's not usually very good.
Many people send one person ahead of the rest of the party to insure a good site for the weekend.
If you are planning on camping in an area that doesn't accept reservations - please call ahead and make sure that sites are available. All parks allow you to stay for 7 days in a row and many allow 14 days in a row so the park you wish to camp in may be full for the weekend before you have finished packing your car. Look out for scams. Sometimes local people will come to the park and take as many sites as they can. We caught a family holding a site 'auction' for six camp sites for July 4th.