Many times, but slipping is usually the least of the associated worries. The biggest problem really seems to be loss of lubrication -- my natural juices can wash away quickly when I'm under running water, and water itself is just not a very good lubricant.
The easiest way has been for me to stand with my hands on the wall with my ass sticking out, and for the man to take me from behind (provided there's enough lubrication for him to get it in). Once that's accomplished, it can be a very enjoyable experience.
Another thing I love to do is give a man head in the shower. Wash his cock and stroke it while he's all soapy, then rinse it, get on my knees, and suck him.
There haven't been any incidents of slipping so far in my experience, but my current lover did pull the shower curtain rod down the very first time I went down on him in the shower...
That was funny. :biggrin1:
Lesson learned: if you're feeling weak in the knees, do not grab the shower curtain for balance. :tongue: