Think about how fattening getting drunk and preventing the hangover is. The dehydration of getting drunk actually stimulates the body to absorb water from the brain, which in fact shrinks a bit. This causes the headaches, dry mouth, and part of the general malaise of the hangover. By drinking water prior to going to sleep a person helps prevent dehydration, and speeds up the flushing of alcohol from the body. Some people even suggest drinking Gatorade or a sports drink before going to bed.
Sugar based drinks can help too.
Again, no caffeine, such as coffee should be avoided as they only act to further dehydrate a person and sleep will be the best escape anyway. Apple juice, Hawaiian Punch, and Gatorade are good drinks. Milk can be hard to digest at this point. If nauseated, clear water at room temperature, taken in small sips will be the easiest to hold down. This helps because alcohol breaks down sugar stores in the liver that need to be replaced.