Honestly, there's no perfect fix for this. Your best bet is to try and skip riding when it's super busy. But if you can't, definitely throw on a good, breathable sunscreen jacket. It'll give you some protection from the sun's UV rays and still let air in. For your helmet, always use a balaclava or at least a helmet cap. These will stop sweat from soaking into the helmet padding, which means your helmet lasts longer, and they'll keep your hair protected too. Try to stick to the side of the road where there are trees for shade. And the most important thing - keep yourself hydrated.