TeamLiquid wiki (http://wiki.teamliquid.net/starcraft2/Mutalisk) tells:
Due to their relatively slow acceleration, Mutalisks should be continually moved at full speed even when there is no harm in sight - this allows them to easily escape at full speed when defenders arrive.
How to do that?
I understand, you need to click continuosly "somewhere" to keep them "flying/moving", not only "flying/standing". But once you click any target they will shoot and stop. Isn't it? Or, immediately after 'shoot' command you need to stack another command "move away" with 'Shift'-click? Or without shift click?