Are cotton underwear better than no underwear for preventing yeast infections?

Probably won't help: Vaginal yeast infections and underwear, tight clothing, etc is mostly urban myth. It may make a difference once in a while, but not in most women. If not yet checked for underlying problems that promote yeast, e.g. diabetes, do that. Assuming no special cause, the best approach may be a once weekly dose of oral fluconazole (Diflucan®), usually very effective in prevention. Discuss with your PCP.