don't do the motions too fast, 720 is all about being subtle, i used to rarely do 720 but now i do j.2C wiff's to 720's and other nonsense.
if you are getting rushed down then back dash 360/720 or block and respond in kind, 720 is a great defensive measure..
when i'm using moves like atomic colider 720 then i start from 9 then hit 8 to finish it but those tricks are for other games...
if you are back dashing then do the 360/720 on the second button press, its so pro its mike z approved.
i can't think of any faster way to do 360/720 just gotta learn to be quick naturally.
oh yeah and im the weakest tager on dustloop, no good damaging combo's just flash.