One of my old coworkers took naps under his desk. He was in a great spot for it: a corner cube facing away from the main runway, so he was pretty much invisible when sleeping even if you were looking for him (e.g. one time I went to ask him a question and thought he wasn't there, but sure enough, he was just napping invisibly under his desk).
If you want to try this, I'd recommend the following actions:
1) Get your boss's approval. Emphasize that naps increase brain power, that you will only nap off the clock, and that you'll be out of sight. Personally, I'd be ok with it if I was your boss, but many people have dogmatic stereotypes about what is and is not acceptable in the workplace, and so if you can't convince him, just forget about it.
2) Never nap on the clock.
3) Make sure you are out of sight when you're napping. This is nice to prevent questions, odd-looks, interruptions, people drawing on your face, etc. If you are blessed with a location like my old coworker, great. If not, consider curtains under your desk. Either way, if someone sees you going down for a nap, don't pretend you aren't going down for a nap or it will look like you are doing something wrong, which if you followed step 1 first, you are not.
4) Use an alarm with headphones. That way, they will wake you up, but only you will hear them and you will not disrupt others.
5) Find the right duration for a power nap or caffeine nap that works for you (probably 15-30 minutes, but no more as then sleep inertia will kick in). The most important thing is that you feel refreshed rather than groggy when you wake up.
6) Know you will be made fun of/talked about. Anything abnormal in the work place is talking material. Even with #3, people will eventually catch on to your noontime-naps. Those points combined mean you will likely be the butt of several jokes (hopefully endearing ones, but possibly malicious depending on your workplace and coworkers). Just make sure you're ok with that up front.
7) Never nap on the clock. Ever.
I think that's about it. Good luck, especially with step 1, and happy napping :)