According to @Wikipedia, a hangover is the experience of various unpleasant physiological effects following heavy consumption of alcoholic beverages. The most commonly reported characteristics of a hangover include headache, nausea, sensitivity to light and noise, lethargy, dysphoria, diarrhea, and thirst, typically after the intoxicating effect of the alcohol begins to wear off. Generally a hangover is experienced the morning after a night of heavy drinking. In addition to the physical symptoms, a hangover may also induce psychological symptoms including heightened feelings of depression and anxiety.
The diuretic effect of alcohol is what causes the drinker to become dehydrated, and it has toxic effects, poisoning many systems of the body. Alcohol is broken down rather quickly and absorbed in the upper portion of the small intestines. It's absorbed into the bloodstream and affects nearly every cell in the liver. Alcohol can affect several parts of the brain, but in general, alcohol contracts brain tissue and depresses the central nervous system. Also, alcohol destroys brain cells. As the kidneys are exposed to alcohol they lose the ability to filter water back into the blood stream. Within twenty minutes of consuming alcohol, a person may experience a need to urinate. This is caused by the disabling of kidneys to recapture water. The water escapes in unequal proportion to electrolytes causing an imbalance.
The best way, I found, to combat the side effects of excessive drinking or completely avoiding them is to HYDRATE. Water is your friend. Water before... water during... water after.