A great deal of the pain of a hangover comes from dehydration. Your body's been working hard to metabolise the alcohol and get the byproducts out of your system - which means pulling water from your body.
So, while there's no cure for a hangover other than time - the best way to reduce the effects of one is to drink water when you wake up and JUST water. Give your body a chance to rehydrate. Drinking more alcohol just makes things worse.
On the plus side, alcohol is an odd energy source. The body can't turn it into fat - which sounds wonderful - drink all you want and you're not going to put on weight... well, sadly no. Beers, for example, are loaded with sugars and carbohydrates along with the alcohol. Guiness makes a point that their beer is almost a meal in a glass.
To make it worse, your body will use the alcohol first because it's easier to metabolise for energy - which means all those other calories you just took in - are going to be used to make fat. So, if you're concerned about weight - stick to the harder liquors with water (not juice) and avoid the fruity mixed cocktails.
The other nice news: up to 2 oz of alcohol a day can actually improve your health. BUT.. more than that and it starts to do more harm than good. Women - sorry - 1.5oz for you. As always, everyone's different and your mileage may vary - but the key is that moderation is the important thing.
Oh... and since a typical bottle of beer is 12oz @ 5% alcohol (up here in Canada, anyway), that's .6oz of alcohol per bottle... so you can have 3 or so per day (2.5 for the ladies) but remember - watch the non-alcohol calories.