If you're planning to drink alcohol- plan the number of drinks you'll have ahead of time. Stick to that number for the night. If you're having a tough time because people are drinking at a faster pace, try ordering a water or soda/juice without the alcohol every now and then to hold your set maximum.
Before you head out, it's important to start with a substantial meal of proteins and carbs in your belly. This will help slow the alcohol absorption process.
Be sure you're mind is in a good place. Drinking in a depressed, angry, or stressed out state of mind can be an unhealthy motivator for throwing back drink after drink which can lead to a hangover, and possibly other unintended outcomes.
Choose your drink carefully. Alcohols produces "cogeners" during the fermentation process. They are responsible for the taste in alcoholic beverages. It's been suggested that the more cogeners in a drink, the worse the hangover is likely to be. Gin and vodka have less cogeners, where as, bourbon and other whiskeys have more cogeners and result in worse hangovers.
And, remember to pace yourself. Try sipping your drink and keeping to about 1 alcoholic beverage an hour. Finished your drink too quickly? Try a cup of water for the hydration your body needs to help avoid a nasty hangover in the morning.
If you choose not to drink, you are ahead of the game without an alcohol induced hangover to be concerned about. If your friends choose to drink, try and provide them with some helpful tips to help them avoid a wicked hangover, as well. They'll thank you in the morning!