eat something beforehand (or eat the shrooms in something, like I did), but make sure it's something easy on your stomach, as shrooms can make you feel queesy and even puke if you have a lot.
also, be carefull when you're drinking water. when I was tripping on them and drinking some, I took one sip where I was really zoned out and I started choking, it was like I was sitting there watching myself choke on the water, and couldn't get myself to move to stop choking. luckily my friend noticed and wacked me on the back to get me going again.
and yeah, listen to the most amazing music you have and it'll be an experience like talking to god himself. If you like metal, I'd suggest Dark Tranquillities "cathode ray sunshine", Rammstein's "Mein Herz Brennt", or In Flames' "My Sweet Shadow".

ps, hope that all came out coherently, I'm quite stoned now, and I badly need chocolate and a friend.