Drink water, a lot of water, before going to bed or passing out.
Alcohol inhibits the production of ADH (anti-diuretic hormone) in the pituitary gland--forcing us drunkards to repeatedly run to the bathroom to relieve ourselves. Of course, we not only excrete away what we've imbibed in the last 30 minutes, but also other essential body moisture and salts. It is this dehydration that causes our brains to physically swell and pound against the insides of our little skulls. So replacing the water you've pissed away during the night will alleviate the most uncomfortable morning-after side effects, though you'll probably have to piss like a racehorse when you come to.