Originally posted by: Whisper
Yep, the above posts have pretty much answered everything. Hangovers are caused by dehydration, so the best way to cure one is to not be dehydrated. I'll usually drink two large cups/bottles of water before going to sleep, and then drink another one after waking up early (which never fails after a night of drinking) along with a couple ibuprofen. 
If you drank a LOT the night before, then at this point you still might feel sort of dizzy/out of it/not quite right. Just get something decent to eat (subway is great), and after an hour or two you'll be ready for another night of par-tay-ing.
		
		
	 
No, a hangover is not caused by dehydration. Dehydration is a by-product symptom.
After you ingest alcohol, your body breaks it down into (among other things) acetaldehyde, before converting it into less harmful substances. The acetaldehyde messes with your brain at the same time as a host of depleted minerals are short-circuiting your nervous system, and that's in addition to low blood sugar and the classic headache-and-dry-mouth symptoms caused by dehydration. The result: nausea, twitchy nerves, unpleasantness, pessimism, terrible brain pain, and a temporary suspension of the laws of gravity.
The severity of a hangover varies according to a.) The amount you've guzzled in a given period of time, b.) Your own innate enzymatic capacity to deal with the poisons, and, c.) Your age.
The more you drink in a short amount of time, the more you'll feel the alcohol. One's weight is also a factor (the less you weigh, the more you'll feel it), as is a genetic predisposition. Finally, the older you get, the more you'll feel the alcohol the next morning.