Now of course the best way to prevent a hangover is not to drink, but the second best is to follow these basic rules from Dr. David Clayton author of "The Healthy Guide to Unhealthy Living" (a great read, for just about anyone so I highly recommend it):

Before You Go Out:

               Eat a Burger- a “fatty” meal stays in your stomach longer than lean protein which slows the absorption of alcohol.

                      Drink 2 Glasses of Water- This lessens the main symptom of hangovers which is dehydration.

While You’re Drinking:

               Avoid Caffeinated Mixers- i.e. Red Bull, Coke, etc… Caffeine can make dehydration worse.

                      Stick to Clear Alcohol- Dark liquors like Brandy or Whisky have more toxins that tend to cause headaches & nausea the next day.

On Your Way Home:

                 Snack-'ems!  Get a snack, preferably one loaded w/ complex carbs.  This helps stave off a dip in your blood sugar which could aggravate any hangover symptoms.

 …Ok, so there it is.  Now that you’re informed don’t ever let me catch you slippin’ or I’ll sing my greatest hits at the highest most irritating octave Amberly possible!  <evil grin>  I wish I woulda had this info about a month ago, sheesh.  Oh well, trial and error.