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?
Answer
As it said in liqupedia
Like all units without firing
animations, Mutalisks can attack while
remaining at full movement speed, if
issued the attack command followed
instantly by the move command.
So, press A-left click-right click
at ground (not unit). If attack cooldown is not elapsed then mutalisks will continue to move futher. Otherwise they attack and continue to move at full speed.
Attribution
Source : Link , Question Author : Budda , Answer Author : Meta