How do you beat the summer heat and keep from getting sick?

I am just curious on how all of you keep cool in this heat? Today I got sick on my horse and scared him silly! (I barfed while riding lol it's funny now, but it wasn't when it happened) How do you keep from getting sick while riding? I just need some ideas on how I can prevent that from happening again. I was kinda embarrassed. :) Any tips?