It's all about being seen! Put reflectors where you can, tape where you can't.

A suggestion from Jobst Brandt: Put strips of good, silver reflective tape on the flat part of your rims between 5 or 6 adjacent spokes, front and rear wheels. Every revolution the wheels "flash" fore and aft. It's pretty noticable.

Be careful not to let the tape get too near the edges, so it doesn't interfere with the brakes.