I usually use a #4 size 1.5" long bucktail clouser in chartreuse-n-white, yellow-n-white or light pink-n-white. I've tried all sorts of sand flea flies, but I seem to have better luck with clousers. I've also caught them on bonefish type gotchas and crazy charlies in a natural tan or shrimp color with sili legs. I'll tie them all up using size "small" lead dumbbell eyes with a clear intermediate sink tip floating fly line or a full intermediate sinking line. That get's them down and puffing sand on the bottom, without going too heavy on the lead eyes, which can be difficult to cast with any kind of distance efficiently.
No matter how you chase them on fly, use a stripping basket to help with fly line management in the surf wave action.