Drink a lot of water. Otherwise you'll be dried out.
There's really nothing you can do to counter the hangover though. What you can do before, is to drink white alcoholic beverages, such as Everlasting, Vodka, Gin etc. since those are more distilled. Its not the alcohol itself that gives you a hangover, its the other acids they contain. And since white drinks such as Vodka are more distilled, they don't contain the same amount of acids, thus it lowers both the chance of getting hungover, and the severeness of it. It also, naturally, depends on person-to-person. Some are more prone to getting hungover then others.
There's several "day-after" rumours to prevent, or at least not make them as bad. They range from raw-egg, to sugar-filled water. The only one I've personally found that works, is drinking lots of water, and having one, just one, drink in the morning. And preferably, a glass of water filled with sugar before going to bed. This makes it so that your blood sugar isn't as low in the morning, and also prevents your body from drying up.