Mixed carbohydrates: combo of simple sugars and maltodextrins. I find at that somewhere between 35 and 50 miles I will get early signs of the bonk. It can be difficults to distinguish between bonk and dehydration, both will make you feel bad.
If you plan a longer ride, eat a good solid meal a couple of hours before and a snack such as Powerbar at the start. Preemptive Powerbar (or snickers for me) at 30-40miles will power me for 10-15miles, then a 100-150 cal carbo snack every 10miles does it for me. I like Fruitopia or similar at 40-60mi, gives 20oz fluid and lots of simple sugars for fast energy- at least 150-200cal. Plan for 1-2oz fluid intake per mile: about half gator/power/acceler*ade, half water with the two oz level for temps above 85-89F.
After the first hour I aim for 200calories/hour. (I weigh 125lbs). If you weight much more than me (most men), aim for 300cal/hour. This will stave of bonking and the miserable drained feeling many have post ride. (You know, the one where you lie in bed exhausted with your heart pounding and can't seem to sleep.) For fluids, it depends on temp, but be sure to get something with sufficient electrolytes. I've started adding drops of "elete water" (http://eletewater.com/) to whatever drink tastes good.