My mage is arcane/fire and I use PowerAura to monitor for the Missile Barrage proc to get my fast Arcane Missiles off (and in her previous incarnation of pure fire mage, I monitored for Hot Streak for the insta-Pyro).
I just pop up a text message with PowerAura telling me which proc has been seen and reminding me which key to use to take advantage of it (I have other procs on other chars, Warlock 'Decimation' for instance, so a reminder is helpful).
If my mage procs, I press 0 which runs a macro on the mage along the lines of...

/stopcasting
/cast Arcane Missiles

and I then go back to my main DPS macro.