Watch what you drink
What you're putting into your system is going to have a strong influence on how you feel the next morning. The more dilute the alcohol the better off you'll be, generally speaking: a good beer drunk is going to be far less painful than a good liquor drunk. On the other hand, the amount of contaminants plays a large part as well. The sulfates present in wine, for example, can lead to especially nasty headaches--especially with red wine and champagne. Vodka comes pretty close to being pure as it's been refined a number of times. Anything aged is usually set in oak casks, as wine and wiskey are, and the tannic acids that give the booze it's mellowness will also give it added toxins.

Drink Plenty of water
Alcohol is something of a toxin, and your liver is working overtime to flush it. Alcohol is also a diuretic, which means that it will tend to dehydrate you. Drinking a few glasses of water just before bedtime will soften the blow the next morning.

Take your vitamins
Vitamins, especially B-12, will help to make the next morning a bit more pleasant. All the alcohol (and the aforementioned flushing) tends to deplete your body of essential nutrients and vitamins. By taking a solid multi-vitamin, you will help to put your body back into balance.

Get a good night's rest
The more rest you get, the better off you will be. Obviously, the more you can sleep, the less you'll have to deal with your hangover. Be sure to go the bathroom before you go to bed, and be sure to take some aspirin, so that your morning headache won't interfere with your sleep. Your REM sleep will be befouled by the alcohol, so prepare to awake somewhat unrested.

The best laid plans of mice and men might not work, however, so read on.