Teepee
While in the shower, soap up your friend. Make a teepee by lacing together your fingers of both hands. Insert said friend in the opening and squeeze to form a seal. Gently pull away from yourself and if the seal is adequate, a suction will be created pulling blood into the penis. Repeat. Repeat. As if by magic, a beautiful erection will result. I've never tried a vacuum pump, but can imagine equal results with the added benefit of fantastic sensations created by the lubrication of the soap. TRY IT. YOU'LL LIKE IT!!