The art of the chair-nap is subtle, but highly skilled. You must be able to maintain a heightened state of awareness (to listen for footsteps) yet nod off enough to achieve a refreshing nap-esque state.
You must also do pre-nap prep: Be able to look immediately attentive, alert, and busy if your chair-nap is interrupted. I keep a highlighter and some important-looking papers handy; that way, if someone surprises you, you can instantly appear to be doing some reading!
If you choose this route, you should also have a work-infused salutation at the ready:
"Oh, hi....no, I'm not busy....just reading this study about cardiovascular interventions in minority populations...what's up?"
Now, there are those who ascribe to the "facing the monitor" version of the chair nap. Those who choose this option have the capacity to chair-nap with hands on the keyboard, so that in the event of chair-nap interruptions, they look like they're writing, designing, reading email, working on a spreadsheet, etc. I could never manage this. I do better with a hard copy document.
For pre-nap prep here, open a document and leave it up. Be sure to adjust your "sleep" mode though, because if you don't touch the document, you don't want your computer to lock you out--a sure sign of not working. The document should be a copy of something important; if you actually fall completely asleep, you don't want to hit a delete key and nuke an important piece of work.
Okay. Enough. I'm exhausted.
Off to chair-nap!