pull the wheel back until there's a lil' slack left in the chain. Tighten down the non drive side. Put your hand right where the chain stays meets the downtube on the non-drive side and press on the rim of your tire, pushing towards the chain. When you have taken out most of the tension using this method, but still leaving enough that the chain is not tight (you do not want tight ... you want to have a little play in the chain or you will destroy your hub / bottom bracket), tighten the drive side. Grab the cranks and the wheel, you want it tight enough that when you move one, the other moves without any slack.