Avoiding Crowds
Don’t camp on major summer holidays. I’ve had very good luck camping right after holidays. For example, arrive at your camping destination on Memorial Day (when everyone else is leaving) and then stay that week.
Getting Good Campsites
To get a good site it is most helpful to do your research. This is especially true of campgrounds you plan on visiting often. If I’m in a campground where I think I might be coming back, I’ll go through the park and make note of the best campsites. Then, the next time I go there I’ll know which sites to ask for. If you do this regularly, in a few years you’ll have a nice database of the best campsites from your favorite parks.