I have played gigs where it was 105+ degrees outdoors ambient, which meant it was 110-125 on stage. Hydration, hydration, hydration. Keep drinking water. Bring a small towel to keep everything wiped down, including perspiration from getting in your eyes. Sunscreen. Then get out of the heat ASAP when done with the gig, determine if you are sunburned in any way; see if you need electrolytes if you are dehydrated, and rest, rest, rest to recover from the gig.