Well, sandpaper is by far the very best way to smooth ABS....IF it's the proper type sandpaper. You want to use higher grit emery based sandpapers. The very best stuff I have found so far is (sold everywhere, including Wal-Mart) 3M emery wet/dry autobody sandpaper. ABS also responds VERY well to wetsanding, done in the same manner as ya would an auto body.
You can also use a sandblaster, but it has to be done just right or it will destroy the print. You can use a very fine glass particulate on VERY low pressure (like seriously low...3-5lbs at the MOST) and very quick passes.
You can use acetone and Q-tips to buff any ABS print smooth. However MANY colors will 'bleach' out a bit using this process, not an issue if ya plan to paint the piece of course. There is also a method using acetone commonly reffered to as VaporBathing.
It's a bit involved so...
https://www.youtube.com/watch?v=biLVyGDvisM
Another option that I have come to use rather alot is heat smoothing. A buddy of mine has a glass shop...I had him make me a set miniature sculpters tools out of Pyrex. I then use a propane torch to heat the tools and work parts of a print. This is not a viable method for working an entire piece, but rather for working details and hard to reach spots. It takes a good bit of practice with each tool to get a feel for how long to heat it. Scraps/misprints are great for this kinda practice runs.