In after effects. I make a solid layer and mask it and animate the mask to a different position and shape for every frame (29 frames per second) for each saber, then add differently feathered layers (I prefer 4 at a 10 20 40 ratio) with a black background behind in a new composition. Then I additive blend the new composition over the old and colorize to whichever hue I want, render, then edit together
Sound like a lot of work? Yeah. It's called rotoscoping. To put in perspective how terrible this is, the following video took exactly 16 hours and 21 minutes (plus or minus a quarter hour or so for a couple times I forgot to stop my stopwatch) to complete:
http://www.youtube.com/watch?v=MoW9Di_A8z4