It's probably because you wake up when you're in REM (http://en.wikipedia.org/wiki/Rapid_eye_movement_sleep). The key is to wake up before you hit REM. I wake up after 30 minutes because I usually will hit REM if I nap for longer.
So do you just always get up after 30 minutes, even if you don't fall asleep for 10 or 15? I find the idea of timing the wakeup difficult otherwise. I guess WakeMate or a similar device might help.
It took me a while, but now one of two things happen during my naps:
1) I don't ever fall asleep. So it's mostly just resting and relaxing with my eyes closed.
2) I fall asleep 10 minutes into the 30 minutes and wake exactly one minute before my alarm rings. My body somehow learned to do this, probably because the alarm sound is so painful. But I can tell that I wake up (I'm not sure who "I" is; it's more of a subconscious thing) right before I fall into a REM sleep.
