I'm sweating too much! HELP?

Tomorrow I'm going to an amusement park and it's supposed to be 85 degrees for the majority of the day, sunny. How can I keep myself from sweating so much, mostly around my head and back. My face starts dripping with sweat, and I get huge stains on my back. I wear a white cotton undershirt, and I'm wearing black tomorrow just to prevent the stains. I know that black makes you hotter, but if I wear green or blue or other colors, I will still sweat enough to get stains. What can I do?