Do you ever have random extruder jams, where it will skip a layer or air-print? You have a nice spring-tensioned, ball bearing extruder, yet it still has occasional jams? This finally fixed it for me.
The filament guide is not the key; there are plenty of those here already. The key is wrapping it with felt soaked in oil. This is the product of a brainstorm with people on the Makerbot google group and some folks at Makerfaire.
This filament guide snaps onto the top of a TOM for use with a top spool. However, I noticed that my PLA filament actually had quite a lot of friction sliding against this PLA guide.
I had also been having random extruder jams, and I noticed on the forums that some people had tried putting a little canola oil on the filament, and that stopped the jamming and didn't hurt the final print.
Someone at Makerfaire had the bright idea of putting those together and using an oil-soaked piece of felt to reduce the friction while concurrently oiling the filament. It works like a charm. I use 3-in-1 oil, since it's what I use on the rest of the bot and it probably has a higher smoke point than canola (you don't want it burning in the extruder). Now instead of oiling the filament twice a day, it's on a continuous feed.