the way i would do it - select the lightsaber with the magic wand tool, or in quick mask mode. Then select the contrast tool in the bottom right hand corner (looks like a circle half black/ half white) go to curves and then mive the curve upwards, you can play about with that until you get the effect you want though.

Photoshop makes some nice glows, but it is very painful if you are wanting to edit a whole movie. in that case I would recommend rebaslight. http://www.rebaslight.com/