1. Sleep. It gives your body time to recover. A tired or unfit drinker is especially vulnerable to hangovers.
2. Water. Keep drinking water.
3. Eat. Complex carbohydrates such as bread and pasta will raise your blood sugar level. Bananas are excellent because they contain complex carbohydrates, potassium and Vitamin C. And if your stomach can't face food? Chamomile tea is best, and make the first cup really strong. The chamomile will help your stomach, and if you take in quantities of water with the tea, it will ease the pain.
4. Exercise. This will help you sweat the alcohol out of your system.
5. Sex. See exercise.
There are doctors who claim hangovers are mostly mental. In that case you may feel the need to punish yourself with a more exotic cures. If so, try the Middle Ages mixture of bitter almonds and raw eel. Or mix together vinegar and raw eggs, and swig them down with a giant gulp.
You might decide you were better off stopping at cure No. 5 above.