What can I use to "sand" my ABS prints?
I have a bunch of solutions to this problem but I'm always looking for additional ideas. I usually start by slicing as much as possible off with a hobby knife. The more than can be removed before sanding the better.

For big prints I like big generic sandpaper sheets from the hardware store. Starting with the highest grit and moving down. Make sure you're discarding your sheets when they get totally gunked up (which can happen pretty fast) or you won't get much done.

For smaller nooks and crannies I have a set of needle files. They do a pretty good job clearing out screw holes and smaller features on printed sculptures.

I've also had some success sanding with my Dremel when it comes to annoying stuck on supports or other imperfections. Having some horsepower behind your sanding can be a nice break. I've also seen the wire brush attachments used for finishing Bronze and Copper-fill prints. Just keep an eye on your speed or you'll sand a hole in your print.

Lastly (and one I haven't tried) are these sanding sticks They were recommended by a friend and look like they would be pretty easy to use/swap out used sandpaper.