I have a bunch of favourite campsites some of which in high season can get pretty full at the weekend. My biggest tip is to take a couple of days off and leave early on Thursday. This way there will generally be plenty of spaces. You can take your pick with the best view for the more secluded spots around the edges.
Over the course of Friday and Saturday you can watch the site fill from the comfort of your own chosen spot.
Also worth considering is taking a few trips before schools and colleges break up , we can have some amazing weather in April, May and June when all sites are still pretty quiet.
Even then if the forecast is good, best to turn up before the weekend. Happy camping!