The best thing I know of is stay in bed and sleep. Do not drink a lot of liquid. Do not eat a lot. Do not take headache medicine. Try and throw up to get it out.
Most hangover symptoms are due to dehydration. You'll want to drink LOTS of fluids if you have a hangover. Rest and ibuprofen will also help alleviate the symptoms. You can also eat simple foods (carbs and dairy) to settle your stomach.
Eat a banana, the potassium will help with the muscle cramps, rehydrate with a combination of your favorite spots drink and water, and treat the pounding headache with your favorite headache medicine. For nausea ginger works well in soda or candy form. Dry toast will readjust the stomach to food.
First, get rehydrated. This can be done with water or juice. Do NOT drink more alcohol to mediate the hangover. Secondly, take a pain reliever. This can be aspirin or tylenol.