Visibility in daytime conditions

To be brief, consider using a neon-lime ANSI Class II or Class III safety vest or another neon-yellow or neon-lime outer layer to catch peoples' attention. I believe high-powered headlights and taillights can be helpful in the daytime too. The photo below shows a rear view of me with a very powerful LED strobe, a Nova BULL LED that's powered by a 1600mAH rechargeable 9.6-volt battery pack intended for radio-controlled cars. I think the light and the neon-lime jersey stand out well in the daytime.

I realize most people won't be hooking Novas to their bike, but consider using a daytime-visible superblinkie such as the Planet Bike SuperFlash, the NiteRider taillight, or the Cateye LD1100 to help catch peoples' eye from a long distance. If you do, make sure you aim the lights carefully, so their "hot spot" is aimed directly into overtaking traffic, not aimed up, down or sideways, or you're wasting your time.