I always keep 1 or 2 of these with me,in addition to the PB Superflash mounted on the bike:
http://www.glowproducts.com/batteryo...esafetylights/

I've never actually needed one,but I'm always handing them out to folks. Usually friends who forgot theirs,sometimes to people I see riding in ninja mode,although alot of them refuse them because they don't want to bother with a light(go figure). They're cheap and I get them in bulk.