A little exercise after your nap, like a few pushups or a run around the block really pushes the grogginess away in my experience. As others pointed out, the timing is key too. 15-20 minute naps works because you are in the pre-sleep state, so you never completely turnoff. However, if you sleep longer, you need to sleep for at least 90 minutes or so, otherwise you will wake up right in the middle of your sleep cycle. I am not sure if that's bad for you, probably not, but it does make you feel like crap.
It's probably a little different with everyone, so practice makes perfect :)