Well, this happens to pretty much everybody. It's just harder to get motivated and energetic during hot weather, and it's harder to do endurance activities because the heat and the sun just suck the energy right out of you. The best thing that you can do is to stay really well hydrated (by drinking a lot of water).
The important part is to keep very well hydrated. If you start feeling weak, dizzy and even nautious (sp?) it may be signs of sun/heat stroke. Head for some shade and drink some fluids (SLOWLY) - if you drink too much too fast after you feel the effects of sun/heat stroke you could make yourself very sick. Water is best, but gatoraid or poweraid will help replace lost minerals.
I had sunstroke last year from building a couple of decks in one day and not drinking enough water. It wasn't pretty.