I find putting an emergen c in one of my water bottles is a big help, that way you are replacing vitamins you are losing. I am also a huge fan of hammer products(the HEED powder works really well during the ride, and the recoverite or chocolate milk afterwards!) also the gels. When you feel like you are starting to bonk have a gel.
If you don't want to buy special substances for the ride, look into trail mix, a banana, or some nuts. They all can be carried in the back pocket of your jersey and work well to keep the energy going.