If you a diving a computer, make sure you know what it looks like and what to do if you *inadvertantly* enter deco. I was on a cattle boat in Cozumel where the dive operator pretty much dictates the profile. For whatever reason, a woman entered deco (was probably penalized for a rapid ascent or something else) and she came up and asked what the beeping ment when she violated her stop. Of course, the computer locked on her but she decided to just start diving with a rented computer (appearently she likes living on the edge).
Some computers will have a simulator, play around with it. Those that don't, read the manually throughly. Not just when you get the thing home but frequently because if its been a year since you've read the manual/played with the simulator, you may not remember what you need to do.