There are two indispensable components of sight casting for saltwater game fish. One, obviously, is the casting, which depends on the skill of the angler and the capability of the rod. The other, equally important, is the seeing: you can be the world's best caster, but you won't catch a thing if you can't spot the fish.
For this reason, the second most important piece of equipment for the sight caster (after the rod) is a good pair of polarized sunglasses. After one session with a guide who sees fish when you can't, you'll understand the benefit of having a high-quality pair of glasses just for fishing. The best that I've come across are made by Orvis. I prefer the nylon aviator model in the Marquesas line.
For most flats fishing (and trout fishing too, for that matter), brown-tinted glasses give you the best contrast. If you wear prescription glasses, get the bifocal model. That way, you won't have to put on a pair of magnifying lenses when it comes time to tie on a fly or extend the leader. Orvis will fill your prescription, or you can order a nonprescription model. Don't overlook the leather side-shields, which really cut down on glare.
Coordinates: $110. Orvis, 800-541-3541; http://www.orvis.com