What helps

Avoid extreme heat or cold.
Drink a lot of fluids and take water bottle/Gatorade/Propel with you when you go out.
Increase salt intake.
Never be by yourself when you go out. (In case you pass out, it's a good idea to have a friend with you that knows what's going on and what to do.
Don't stand up quickly.
Don't take hot baths or soak in hot tubs.
Try not to stand for too long, if you have to, move your legs around so the blood won't pool in your legs.

normal leg--> <--pooling