i also felt a lot happier about my visibility when i had my tireflies in. they're little motion-activated single-led flashers that you screw into the valve stem of your tires.
http://www.mec.ca/Products/product_d...=1127435953499
of course they're no help head-on or when your tires are below drivers' range of visibility, but pretty hard to miss otherwise. i felt like such a dork when i had them, i figured i was probably pretty visible.