I do not think it is possible to be too well seen at night. In my opinion, being seen falls into two categories: passive (reflective gear) and active (lighting). I am amazed that so many cyclists are willing to put their lives at risk by not making a better effort to be seen by motor vehicles at night.
In addition a good tail light on my saddle bag (and a lot of reflective clothing) I also use a Planet Bike Blinky 3H Tail Light mounted on my helmet. The helmet mount is self-leveling which means it will always be visible to traffic directly behind you, regardless of whether you (and your helmet) are looking up or down. This light has three bright LEDs and a built-in parabolic reflector. In flash mode the light should run for about 100 hours on a single AAA battery. You can also run the light in continuous mode, but I think a blinking light draws a lot more attention. The Planet Bike web site claims the Blinky 3H can be seen from up to a mile away.
The Blinky 3H comes with both a helmet mount and a seat post mount. The helmet mount is Velcro and you can attach it to your helmet in under a minute. The list price is $23.00, but I got mine from Amazon.com for $18.00.