For me, the best thing I ever started doing was to bring one of those shower poof things. They are cheap so I don't worry about bringing it home with me. I never used to really feel clean after a day at the beach before; I couldn't seem to get all of the sunscreen off with just a wash cloth. Now I bring a poof and it works out way better.
Also a bunch of extra zip lock bags. My beach bag is packed every day with everything I need in various zip lock bags - books, cameras, iPods. Helps to keep them dry and sand-free when not in use.
First aid items that cost Way too much in the gift shop: bandaids, neosporin, excedrin,Pepto (tablets),insect repellant wipes,robitussen caplets,imodium AD, and any other OTC drug you might want to take.