I also made an example applet with a billboarded-quad glow effect on
this thread, and I posted the source code there for reference. The basic idea works like this:
For the billboard itself, make it the same diameter as the longest diagonal length of whatever object is supposed to be glowing (such as the car headlight in Egon's example). Then, offset the two polys of the billboard to half that distance in the -z direction in object-space. Finally, place the center of the billboard at the center-of-mass of the headlight.
Here is a diagram in case my description above doesn't make any sense:
One additional thing that I added besides the glow quad is an actual light source so the glowing thing also lights up surrounding objects. I commented the code pretty well, but if you need help understanding anything, let me know.
I have also been playing around with other special effects for a target-shooter game I'm working on, including motion blur, gun-flashes, energy shields, smoke, and explosions. If you have any trouble with effects like these, I'll be happy to help.