Don't use the handicapped stall.
Unless, of course, you happen to be a handicapped person in which case you are one of a select few that should use it. Almost every time I go into the washroom at work the handicapped stall is occupied. Whenever I see somebody come out of it, he* is an able-bodied, non-handicapped coworker. There's something very wrong here.
The handicapped stall ought to be thought of the same way that the handicapped parking space is. Actually, I take that back. The handicapped stall is more rare than the handicapped parking space. If I see one handicapped space, I see at least two next to it. If I see a handicapped washroom stall, I see only one, and it's tucked away in the corner of the washroom.
You shouldn't park in a handicapped parking space if you aren't handicapped. By that same token you shouldn't use a handicapped washroom stall if you aren't handicapped. That stall was put there so that a handicapped person can use the facilities easier. The stall is larger to facilitate a wheelchair, not so that non-handicapped people can have more room if the stall is unoccupied.
Just because there's no police officer to dole out tickets and fines doesn't mean something is okay. So you haven't been caught yet? How will you feel when you exit that spacious, roomy stall and there's a person in a wheelchair looking very uncomfortable because you've selfishly taken the only stall s/he can use?
...some time later...
After an excellent /msg conversation with yclept, it occurs to me that I should make myself a bit clearer. I think of the handicapped stall as a commodity. The washroom I regularly use at work has six urinals and eight stalls. I have never seen every stall occupied, but the handicapped stall is almost always occupied. My thoughts on the situation would be different if the washroom that prompted this writeup were a single-user affair that was handicapped-accesible. Also, I understand that there are times when the handicapped stall is the only one available, and sometimes you've just got to go. Just don't use the only stall available to a handicapped person if you can help it. I'm sure they'll appreciate it.
* I have only ever seen the interior of a men's washroom. I assume the observations made here also apply to a women's washroom.
In response to some of the softlinks, I am not handicapped myself. I just find it amazing how rude we can be when we think nobody will ever notice.