1. Right before you crash, down a cup of java. The caffeine has to travel through your gastro-intestinal tract, giving you time to nap before it kicks in.
2. Close your eyes and relax. Even if you only doze, you'll get what's known as effective microsleep, or momentary lapses of wakefulness.
3. Limit your nap to 15 minutes. A half hour can lead to sleep inertia, or the spinning down of the brain's prefrontal cortex, which handles functions like judgment. This gray matter can take 30 minutes to reboot.
A caffeine nap is a short nap that is preceded by the intake of caffeine. In a driving simulator and a series of studies, Horne and Reyner investigated the effects of cold air, radio, a break with no nap, a nap, caffeine pill vs. placebo and a short nap preceded by caffeine on mildly sleep-deprived subjects. The last mentioned was by far the most effective in reducing driving "incidents" and subjective sleepiness. Caffeine in coffee takes up to half an hour to have an alerting effect, hence "a short (<15min) nap will not be compromised if it is taken immediately after the coffee.