Cleaning your hot end should be a regular procedure. The best way I've found to do it is to heat up the hot end to a few degrees over the temperature you would normally print at, let it settle, then reduce the temperature to just above the glass transition temperature of the plastic you're working with. About 90 Celsius works best for me with the PLA I use. Once the lower temperature has settled, back the filament out of the printer. As per jimc, you should see a straw at the end of the filament you backed out, which means that the plastic has been dragged out of the hot end. It should bring any debris / old plastic out with it. Repeat as necessary until you're happy and make it part of your regular maintenance schedule.