Here's mine - it has saved my a$$ many times at many homebrew competitions where I was walking the next morning when I should have been crawling:
BEFORE you go to bed take:
4 x 200mg ibuprofen
1 x zantac, tagamet, tums (whatever you normally take for stomach acid issues)
1-2 x Benadryl
and a big glass of water (at least 8 oz)
As it was explained to me, yes, a major component of the hangover is dehydration (thus the big glass of water); but, in addition, it is also an allergic reaction by your body to the alcohol (a toxin). Your body reacts to the excess alcohol and releases histamine in your tissues, brain and blood. This is why you physically hurt the day after - and why the IV previous posters have mentioned helps so dramatically. You are not only hydrating your body, you are also diluting the concentration of histamine in your blood and helping the rest of your body flush out or reabsorb the remaining histamine.
For you med types out there, here is an abstract from a study of the alcohol-histamine interaction from a study on this done about 12 years ago:
Alcohol and histamine metabolic pathways in the body have the common enzymes aldehyde dehydrogenase and aldehyde oxidase. The metabolite of ethanol, acetaldehyde, can effectively compete with the metabolites of histamine, methylimidazole acetaldehyde, and imidazole acetaldehyde. At the periphery, alcohol and acetaldehyde liberate histamine from its store in mast cells and depress histamine elimination by inhibiting diamine oxidase, resulting in elevated histamine levels in tissues. Histamine mediates alcohol-induced gastric and intestinal damage and bronchial asthma as well as flushing in Orientals. On the other hand, alcohol provokes food-induced histaminosis and histamine intolerance, which is an epidemiological problem. There are many controversial reports concerning the effect of H2 receptor antagonists on ethanol metabolism and the activity of alcohol dehydrogenase in the stomach. In addition, alcohol affects histamine levels in the brain by modulating histamine synthesis, release, and turnover. Histamine receptor antagonists can affect ethanol metabolism and change the sensitivity of animals to the hypnotic effects of alcohol. In contrast to other neurotransmitters, the involvement of the brain histamine system in the mechanisms of the central actions of alcohol and in the pathogenesis of alcoholism is poorly studied and understood.