You can buy track nuts with built-in floating serrated or smooth washers. When you tighten the nut, the washer does not turn and gouge the dropout. However, I've also used heavy duty smooth flat washers with separate standard hex nuts on my road fixies without any slippage problems. It is easy to tighten them sufficiently to obtain the necessary friction connection without stripping the axle or nut threads. Standard serrated flange nuts are the worst; not only will they gouge your dropout, they will make it almost impossible to properly adjust your chain, as the axle will move in and out of the dropout as you tighten the nut. Therefore, if you don't want to spend the $$ for some high end track nuts, you are better off just using standard hex nuts and heavy duty washers.

http://www.sheldonbrown.com/harris/fixed.html#bolts