What I've learned this season (the hard way):
1. Off-season training is paramount.
2. Keep hydrated with a high-quality electrolyte replacement (I use Nuun tablets)
3. Keep hydrating with (2) after your ride, your body is still depleting your stores long after you stop. A couple of weeks ago, I had a post-8hr-ride leg cramp that almost brought me to tears.
4. Gatorade doesn't have enough electrolytes for me, gets me sick to my stomach, and discourages water consumption. (see #2)
5. Keep adjusting your approach until you find what works. Lots of people recommend E-load, and such. I've tried it and had the same problem as Gatorade.