Prevention is better than cure, they say. It's all well and good curing a hangover, but wouldn't it be better not to have it in the first place?

[edit] Steps

Lots and lots of beer...it can be a bad idea.

Avoid alcohol. Of course, the need to prevent a hangover wouldn't exist if alcohol was avoided completely. Just realize that excessive drinking of alcohol is what causes most hangovers. The following steps assume that you are planning on a night of drinking.
Eat a good dinner before you go out. The food will absorb some of the alcohol you are about to drink.
Choose clear alcohol like vodka, white wine, and light rum instead of red wine, whiskey, brandy, and sherry. Similarly the dark beers have more consequences the morning after because lite beers contain more water. Avoid sugary drinks or malt beverages.
Cheap alcohol is never a good idea. If you have the choice, always go for "Call" or "Top Shelf" drinks rather than "Well". A "Well" drink is the no-name brand or "House" brand the bar gets at a discount.
Continue eating as you drink. Fats and carbohydrates will put a brake on alcohol absorption, while eating sugars will replace the sugar stores that the booze removes from your liver.
Drink a glass of water after every alcoholic drink. Alcohol always dehydrates your body, which in turn causes the headaches and other aches you feel during a hangover. One glass of water in between every glass of alcohol is a good principle.
Know your limits. Avoid getting into buying rounds among your friends, as this will just encourage you to drink more. Instead, just buy your own drinks at your own pace, or agree to a certain budget with your companions.
Drink another glass or two of water when you return home, and, if possible, have something else to eat, such as a banana or two.
Take a multivitamin or liver salts before bed to replace the vitamins and minerals lost as a result of drinking alcohol, and drink plenty of water to rehydrate yourself. If you ate a good meal before going out, this step may not be necessary, but to prevent any potential hangover, it can be better safe than sorry.
- When you attempt to go to sleep, if the room is "spinning" when you close your eyes, stay awake until the effect goes away, or else you increase the chances of getting sick. You may also find that hanging one leg off the side of the bed and placing your foot flat on the floor helps to reduce the effect.
- If you feel a hangover coming on in the morning, eat something and take some Ibuprofen, drink several large glasses of water, and go back to sleep.
Drink slowly. Your body burns alcohol at a regular rate of roughly 30ml (1oz) an hour. Give it more time to burn that alcohol and less will reach your brain.

[edit] Tips

In terms of amount of alcohol consumed, 12 oz of beer = 5 oz of wine = 1.5 oz of spirits. Don't think you're drinking less because you're drinking white wine instead of Jack Daniels and Coke.
Keep a large bottle of water by the bed you're crashing/sleeping on. When you'll wake up during the night or in the morning, you will be craving for water, but the headaches or someone throwing up in the bathroom will keep you from getting up or walking to get your water. Water by the bed is comfortable and makes the morning a lot more pleasant.
Cheese and nuts are good foods to eat while drinking because the high fat content slows the absorption of alcohol. Bar snacks can be your friends.
Avoid smoking while drinking. Smoking constricts your lungs and decreases the oxygen flow to your blood stream. Less oxygen means a stronger hangover.(But at the same time if in need can be used while drinking to reduce the effects of alcohol if you find that you had a little to much to drink.)
Vitamin B helps replace the nutrients lost while drinking. Take a B-complex vitamin before you start drinking and another before you go to sleep.
A good source of vitamin B is brewer's yeast, which is in home-brewed beer and bottled conditioned beers. Therefore, it has been suggested that drinking these varieties might give you less of a hangover. It can also be found in some energy drinks like Red Bull. See "Warnings" below for problems that may arise when mixing alcohol and energy drinks.
When deciding how much to drink, always take the ratio of stomach contents (food you've eaten) to alcohol into consideration. The more you've eaten, the more you can drink without feeling light-headed.
Avoid caffeine and anything that will cause dehydration, such as coffee, tea, or soft drinks.
Sports drinks will hydrate you better than water, and can also double as a drink mix for hard alcohol.
When eating prior to drinking, food high in complex fat and protein -- like a big steak -- work better at absorbing the alcohol.
When all else fails, try "Chaser" -- a hangover prevention drug -- which can help in absorbing congeners from the alcohol in your stomach. See "Warnings" below about this drug.

[edit] Warnings

Don't drink on an empty stomach.
Don't take ibuprofen on an empty stomach.
Never take acetaminophen/paracetamol (Tylenol) before, during, or after drinking.
Don't drink and drive. If you are planning to drink, have a designated driver handy.
Just because you've taken preventative steps against hangovers, that doesn't mean you won't get drunk. Always drink responsibly.
Always read the label on vitamins or any other drugs, especially the health warnings, to ensure that there will be no adverse side effects when mixed with alcohol.
Be careful when consuming alcohol and energy drinks (for vitamin B). Too much energy drink mixed with too much alcohol can lead to a severe, and possibly fatal, increase in heartbeat.
Using "Chaser" or any other congener-blocking drug does not prevent individuals from getting drunk. They only prevent or lessen the effects of a hangover.
While consuming fatty foods and fat may help to prevent a hangover, alcohol consumption combined with fat consumption will lead to increased weight gain/fat mass and perhaps a fatty liver. A fatty liver may be much less able to metabolize alcohol. When alcohol is metabolized it is readily converted into body fat, and is very high in calories. Your liver will metabolize alcohol before it ultilizes dietary fat. The dietary fat will be stored as fat tissue instead of being burned as energy, and the high-calorie alcohol will most likely also add to this fat. And since heavier people can drink more before getting intoxicated, the weight gain may be compounded over time.