If you can, pick up as many disposable cold packs from a pharmacy as you can. There's ones you can shake and they get cold for a while.
Hand fans are your friend. So is breezy clothing, don't wear synthetic fibers if you can avoid it, they don't breathe a bit.
Drink drink drink. As much gatorade as you can manage- The better your PoTS is managed, the easier the heat will be.
Try and get a water bottle made to stay one temperature. And fill it to the brim with ice whenever you can.
Wear compression socks. It may make you warmer for a bit, but it will lessen the effects of your PoTS.
And the MOST IMPORTANT THING: If you're feeling ill, IMMEDIATELY TRY TO COOL OFF. Don't try and stick it out. Us Potsies are far more susceptible to things like heat stroke. If you start to feel light headed, get to a safe place and lie down.
The reason we feel so bad in the heat is that heat relaxes us. It opens up our blood vessels more, making our blood move even worse than it already does. Treat the PoTS, and the heat will get easier.
TL;DR: Cold packs from a pharmacy, hand fans, light natural-fiber clothing, hydrate, compression socks. And remember: SAFETY. COMES. FIRST. If you feel too sick, if you feel light headed/dizzy, go lay down and try to cool off.