I've been on a quest to improve the print quality on my MendelMax 2.0 printer. Watching a Replicator 2, I've noticed the fan makes a huge difference. I had been using two fans angled toward the top, using these mounts: http://www.thingiverse.com/thing:139952. You can see the latest version of this mount on the right side of the photo.
In an effort to improve the cooling airflow, I created this fan duct, which screws onto the carriage. This fan duct is thicker than many you'll find, but does a much better job. Here's why. The fan supplied by Makers Tool Works for the MendelMax 2.0 is good at moving air, but it can't pressurize the air much. So if the output vents are too small, most of the air flows back out of the fan on the outside of the blades rather than flowing across the part you're trying to cool. Using larger vents allows more air to flow around the part.
With this duct I've gotten the best print quality so far with my printer. Bridging works much better, and I get less sag on overhangs.
The last photo shows an example of before and after switching to using the fan duct. The light blue part is without the fan duct, while the dark blue part on the right is with this fan duct. You can see how much of a difference it makes. Both of these parts were printed from exactly the same g-code file.