Here's a simple Thing for cleaning the burnt ABS plastic off your nozzles. Best part, it's reusable!

This is based off the idea of Jetty's MK8/MK7 Nozzle Cleaning script:
http://www.thingiverse.com/thing:30454.

However, instead of using a script, you just shove this Thing on to the nozzle after a print and wait until the nozzle cools to about 60-70c. Then, just twist/bend and this Thing should pop right off... along with all the burnt gunk!

I usually embed the nozzle just a hair past the darkened plastic bits on the nozzle, so removal should exert minimal force on the mechanics of your 3D printer.

Note/Disclaimer: This does require getting close to a piping hot extruder nozzle with your fingers, so be careful!