I do mine in the sink. I'm mildly compulsive about brushing my teeth, and usually end up brushing them about five times a day, so it's most convenient to just use my toothbrush in the sink. When I lived in a dorm my freshman year of college, I brushed my teeth in the shower in the morning and then in the sink at night. The sinks were always really busy in the morning, so I saved time by doing it in the shower.