Some things that I've started doing that seem to help tremendously, especially when working behind the plate.
First of all, I invested in these:
RiteTemp Athletics Officially Cool Endurance Pad Set: Ump Attire
I can't say enough how well they work and keep the heat manageable. That furnace feeling from under the chest protector goes away with these, I kid you not.
Keep well hydrated obviously but before each game, I'd drink a bottle of coconut water which is naturally sugar free and loaded with electrolytes. This helped prevent cramping and helped ensure a proper electrolyte level.