I agree with Anthony about snorkeling along the edge of the lip -- that's always turned them up for me. We visit only once or twice per year, but I find great shells every time and have a wide-ranging collection from just the past 4-5 years. I find scotch bonnets (my favorite) washed up on the beach every time we visit Seagrove, for some reason. Found tons after a good storm, too, but I had to net them in the surf in the early morning before the other beachcombers woke up.
I've heard many people say you can't find shells on 30-A beaches, but I always do -- they're there if you look...
(Oh, and if the June grass is onshore, look for sea beans, too. Found some of those last year, including my first "hamburger bean." Cute.)