Body powder. After showering and drying off, pat some on each thigh and go.
I went to New Orleans in August a several years ago and only wore skirts and shorts since anything else was brutally hot. Ignored advice to put on some Gold Bond powder on my legs, and a day of walking around the French Quarter in a skirt had my thighs chafed so bad I was bleeding and peeling skin!
I used the powder every day for the rest of our vacation and it was the only reason I could still walk. Even in 100% humidity and 90 degree weather, walking all day, it kept me dry.