I've been using Dunlop T3's for a while now because I prefer the attack of a sharp pick, but they always go dull fairly quickly. I've tried filing them, carving with knives, cutting with scissors, and just end up breaking them. Turns out all you need to do to sharpen your picks is drag them across the carpet a couple of times! Not only does it resharpen sharp picks, but it sharpens normal rounded picks as well! So far I've only confirmed it works on Tortex, not Ultex or nylon. For reference, the two purple picks were completely normal, rounded picks. I dragged them across the carpet three times each side, and bam.
I used to do that when I was using Fender picks, but I never tried it when I switched to Dunlop Max Grips, which are nylon. It actually does work on them, although you have to do it more than once.
I just use a fine grit finger nail file... But I should point out that I rarely if ever use picks on bass. Only guitar. I like to keep them silky smooth and often put speed bevels on them.
I tried sharpening my picks years ago but it doesn't really work for me. If they're too short, it doesn't make a difference to me whether they're sharp or not. I do a lot of tremolo picking on guitar (orange Tortex) and I can easily go through two picks at a rehearsal or recording session. With bass I use thicker picks (yellow Tortex), tend to strike the strings flat rather than with the edge at an angle, and don't tremolo pick, and they last me a lot longer.