Cold Pull
Heat the Nozzle to the Print Temperature of the Material you are using.
Extrude some material to verify it can flow from the nozzle.
Cool the Temperature down to the Materials Rubbery State. For most PLAs, 90°C works well.
As soon as the nozzle reaches the Rubbery State temperature, quickly pull the filament out
Because there might be slight differences between filament manufactures you might need to adjust the temperature of the pull. If it breaks off, raise by 5°C, if it stretches too much, decrease by 5°C.
Also see Clogged Nozzle.
Filament Cold Pull Temperatures:
PLA - 90°C
ABS - 170°C
PETG - 130°C