Saph has really good tips in their post! Personally the biggest things I have found are the visibility (my toony suit has pretty great vision, but can't see anything below my nose, and my dragon suit I can barely see out of) drinking water, and learning your limits. I have a pretty high heat tolerance, but I make myself sit and blow a fan on me every hour or two to keep myself cool, cause once your body heats up, it heats up quick. Depending on the arrangmet of your suit, a water bladder with a hose is a great way to get easy access to water, but I can't use one so I made water bottle holsters i attach to a belt or harness (doesn't take away from the suit, I adorn my belt with shinies) and use a straw to drink.
I don't use underarmor o a wetsuit, I personally just use long activewear yoga pants and an activewear longsleeve shirt. I use a wig cap and a balaclava in my head to hide my mouth and add a layer between my skin and the foam or resin.