Don't litter.
Use a camel back (but wear a shirt under it! I learned that lesson today) or a fuelt belt.
Run where there are water fountains along the way.
Wear sunscreen.
Use gells. My current fav is accel gel because it has protein. You can get them cheapest online a whole box for around 18. They cost 2-3x as much at stores. That goes for pretty much any gel.
On hot days, I put ice into my camel back and it melts as I run. Fuel belts don't hold as much, but you can put stuff other than water in them (Gatorade, etc.) - Camels are water only. So use both.
One last rule, probably the most important (after no litering) - run EARLY when it isn't as hot.