Two nasty side effects of alcohol consumption are dehydration and acetaldehyde build-up in your system. Both of these can lead to the hangover symptoms. Dehydration’s obvious cure is to drink as much water as you can while consuming alcohol. Acetaldehyde is a toxin produced as your body metabolizes the alcohol. You body can process it, but only so much at a time. A build-up will definitely give you a nasty hangover. There are a few supplements that can help tremendously with this breakdown process, here are a few:
http://www.pretoxx.com/hangover-cure.html
http://www.lef.org <- search for anti-alcohol antioxidants
Also, some suggest eating raw eggs before drinking. This is similar to the supplements above—they all provide the body with cysteine. It is cysteine that assists to breakdown the acetaldehyde.