My nozzle was getting covered with the burnt remains of past prints and I couldn't think of any easy way to clean it. When I saw this suggestion on the Makerbot Google group my initial reaction was "Naaaa, that can't be a good idea." But it is, and it works.
Get a blob of a past failed print. If you've got something that's a solid blob of plastic (or 100% infill) use that. Heat up the extruder (I do this after changing filament so that it's already hot) and melt the blob on to the extruder head. Bury as much of the head as you dare in the plastic. Then let the extruder head cool. When it's cool get your needle nosed pliers and yank the cooled blob off.