Autoattack cancels casts??

I'm a Lvl 18 cleric atm using mainly the magic dmg chain but also the physical chain on cd plus autoattack.
And it's really annoying that autoattack rupts my own casts ...

Any suggestions how to avoid this?

Or am i doin it wrong somehow? :P

just dont press anything else in the beginning of the cast or it will cancel, near the end you can even move a few steps and it wont cancel.