Keith of Keith’s Electronics Blog added a fan to his printer to regulate how the fabjects cooled and shrank during the print process.
I connected a spare DC fan to my bench power supply, set it just outside the CupCake’s build chamber, and dialed it down until it barely spun. Holy schmoly! The left cap (you’ll need to click the image for the larger version), made with no cooling, looks like an art project woven out of twigs; the next one is extremely smooth on the side that was facing the fan — better than anything I’ve printed before!
Keith gets extra cred for printing his own fan holder (top pic) — you can find similar things on Thingiverse.