I gues it all depends on the look you want. For a basic shot, I leave the shower door open and shoot the image with a 36x48 softbox pointed through the door. Since my shower is off-white tile, the "bounce" is enough to evenly light the model.
Here's an image I shot in my shower with one big softbox.
For a more "produced" look I like to add a hairlight/backlight. Since my shower has glass doors that don't go all the way to the top of the shower (this technique also works in showers with shower curtains) I place my Nikon SB-800 speedlight (battery operated) on a boom arm and run it over the door and above the model and as high up as possible to keep it dry (a pastic bag can be used to protect the speedlight).