Start casting, then cancel around 40% and use an instant spell.
You´ll have to learn when they interrupt and how often.
Wave your arms around and look angry.
I get halfway through the cast, move, then cast a real one, then do it again in case they didn't silence the first one
download interruptbar so you know when they interrupted and how long until you have to fake-cast again
This sucks in lower brackets and battlegrounds though. People are not experienced and interrupts will be pretty random.
Best bet is to hardcast a slow spell by purpose of a different spellschool and see if people interupt it. Arcaneblast for frostmage for example.
If people interupt, fakecast, otherwise, just hardcast.
To fakecast, just start casting a spell and around 30%-40% you interupt your own cast by moving, jumping, etc... When you do that, don't start casting a new spell immediately. Because of lagg someone might interupt your real cast instead of your fakecast.
I do this from time to time on my mage except I use fireball instead of AB. Getting the arcane tree locked out is bad news. At lower brackets people will interrupt nearly any cast they are able to so I might get silenced for a couple of seconds but otherwise it was a wasted interrupt. I sometimes opt for this over fake-casting because I'm too impatient and make it obvious that I'm fake casting.