First and formost would be to stay hydrated. Maybe fans in the helmet and under armor type clothes under the costume. Even if you do all that, it's still going to be hot, one would just have to try to figure ways to minimize how hot they get by taking breaks.
Boba helmets have a lot more airflow underneath than say Stormtrooper helmets. I've been trooping my Boba for almost 14 years and haven't put a CPU fan in there yet. With other helmets, yes, you need it. Like Jc27 said.....stay hydrated before your event, and on breaks drink more water, and then when done trooping, drink even more. You just have to know going in you're going to get hot and sweaty regardless. That's just a normal part of costuming. LOL
Though I haven't built my kit yet, ner vode on mando a.org suggested putting ice packs in the back of your under suit down the spine and wrapping copper wire around your torso, making sure it comes into contact with the ice packs. Copper is an extremely good conductor of heat. Or, if you don't mind having heavier and clunkier kit, have a long, skinny tube of water that runs through your arms, legs, and torso, periodically protruding from your suit. Attach both ends of the tube to a small electric pump, and you have a cooling system. It cools you down the same way a dog pants. The water gets cooled when the tube protrudes from the suit. Hope this helped!