bam:
http://www.identi-tape.com/reflective_gateway.htm
I put a strip on the back of all my fenders. Usually put strips on the chain and seatstays too. Can even color-match to your bike.
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C'dale BBU('05 and '09)/Super Six/Hooligan8and 3,Kona Dew Deluxe,Novara Buzz/Safari,Surly Big Dummy,Marin Pt Reyes,Giant Defy 1,Schwinn DBX SuperSport,Dahon Speed Pro TT,Brompton S6L/S2E-X