Side vis is very important, and that's why I chose the lights I did.

The headlight is a Philips Saferide, this light has a glowing ring that is easily visible from the side, plus it throws out a beam to about 80 degrees to your side and far enough to go across two lanes so any car coming behind you will (or hopefully will) see the beam projected onto the pavement like a motorcycle light does.

I use several tail lights but the best one I have for side vis is the Light & Motion VIS180 (not the Vis180 Micro), this light has two amber side lights that constantly flash, but even the red part can be seen pretty good from the side. Also, like the Philips, it casts a red halo onto the rear of the ground behind you. I also use Soma Road Flares in the barends and these too can be seen quite easily from the side. And the last light with some side vis is the Cateye LD600, this light if mounted vertically is very impressive for side vis but the due to better LED technology the rearward is not up to the new stuff, if you mount it horizontally then it reduces the effectiveness of the side vis by quite a bit. I also use a Planet Bike Turbo Superflash on the helmet but the side vis is poor on this one compared to the others.

Also don't discount but don't rely on reflective material. I use reflective leg bands which are vis from the sides, front and rear; I also wear a lime green safety vest that cost me $8 at Home Depot, it has wide reflective strips and it's mesh so air flows through it. I put reflective tape on the helmet but it doesn't really work good due to the angle of car headlights vs how high the helmet is.