--- Note ---
I learned the hard way, when printing ABS, turn the fan off altogether. It was causing part curl at the end of the print.
--- Update ---
I installed a 10uf capacitor across the (+) (-) terminals and it quieted the motor down substantially. For the heck of it I tried a 100uf cap, and I lost the ability to adjust the speed of the fan, it's like Ramps wanted to push the fan 100% on with no speed control. So somewhere between 10uf and less than 100uf and you should be good. Picture added.
This is a mashup of 3 existent "things", the parts from which I'm using as a print cooler that works well with the RepRapGuru hot end assembly.
Print these three things and add to it a 5015 12v blower fan. I zip-tied my clip to the extruder motor for extra security.
Note: this produces a LOT of airflow... enough to affect print bed temp and make the printer work harder to keep the print head heated. You'll want to experiment with the fan % in your Slic3r config. I wonder if a 24v fan would work better since it would have a lower output when fed 12v.
Driving the blower motor at ~30% works nice, but it's LOUD. I'm going to put a 10uf capacitor across the fan power leads to try and quiet it down. PWM is so noisy. ;)