For a while i have had this problem when i'm testing my arcane DPS on a training dummy... But as arcane you can never really properly test your dps this way.
A big part about arcane is nukage, and atm with mana regen the way it is, we can just Nuke off the bat until we get to 30% Mana and then evocate and do the regular mana conservation from there.
But the thing is, when we Evocate we have to automatically cancel it at 1 or 2 ticks of mana regen (Loss of about 30% max mana returned) before we automatically leave combat.
One way i have gone around this is by popping Mirror Images right before i evocate, since they keep shooting stuff, keeping me in combat.
Now that i have gotten my t10 4 piece bonus though (sad, i know), i really would rather Not spend my 30sec 18% Damage increase CD on using a 4-5 second channeling spell to regen mana and then a basic weak rotation. If only training Dummies would keep you in combat longer =(
I'm normally able to stay in combat by just flicking off a quick ABar at the end of the complete Evocate.
If for latency reasons you can't do this just bring a friend in your group to keep hitting it and keeping you in combat