Ukon is supposed to be good. Apparently you drink it before you start boozing, to line the stomach and cushion you from the alcohol. Though I'd have thought you might need a few of them if you're going to be drinking a lot.
To pre-empt hangovers, I find drinking cocktails mixed with plenty of fruit juice is good, vodka orange and the like. The hangovers usually aren't so bad, I guess because of the extra vitamins. Green tea and fresh juicy fruit the next day, like Japanese nashi (pears), has worked for me in the past too.