Building on Coyote’s post, it’s much better to take the milk thistle before you go out drinking – take a double or triple dose and it will help stave off liver damage (I have no idea whether it will help the morning after).
Also, a strong second for the NO ACETOMENOPHEN (Tylenol) rule – when you process it you make a byproduct toxic to your liver. It’s easy enough to take too much when you haven’t been drinking, let alone when your liver is already taxed.
Obviously, the best way to avoid damage is to drink a lot of water while drinking alcohol and minimize your consumption, but we all go overboard occasionally. Here’s the best anti-hangover prevention I’ve found (and back in my college days I had my share of benders), for use after you get home and think, “Oh God, what have I done to myself now?”:
1 Quart Gatorade
1 quality multi-vitamin
2 Tablespoons of peanut butter (I’m sure Coyote’s fish oil would work just as well, as you really are after the fat)
If you’re sober enough to take that when you get home, you’ll thank me in the morning.
And as everyone else basically said, don’t worry too much about the damage – just train hard and watch your diet extra closely for the next week and you’ll be all set.