Various hypotheses have come forth to explain why we have a disproportionate number of ideas and epiphanies in the shower. Explanations as varied as guessing that it's due to the aromatherapy of shampoos, or the brain soothing effect of warm water on our scalp.
I have a simpler explanation: It's one of the very few times when we actually disengage from the world and think. Lathering and rinsing tends to be a very automatic activity, and given that we don't have the option of reading the paper or browsing websites, we're forced to converse with, and be entertained by, our own mind.
A similar result can be experience on your daily commute (this
only really works if you drive by yourself, presuming that the
traffic isn't infuriating and distracting. I've found commuter
trains, buses and carpooling to have no such meditative effect, and
instead to be highly detrimental to thought): Turn off the
radio and just relax on the drive.
It is astounding how many decisions and ideas are resolved on a 30 minute commute in this island of tranquility. It is the activity closest to meditation in my daily schedule.