I focus on the tank, and then create a macro for all my attacks that start with:
/assist [@focus, exists] (this will attack what ever my focus is attacking no matter what i have targeted at the time)
on the macro's for my attacks, if the attack is important, when it procs, I add to the macro
/stopcasting (this will stop any attack and activate the attack I have set in the macro)
be aware that the focus may need to be turned off for some bosses. The rogue thread has another macro for setting a focus and target for tricks of the trade, which is similar to misdirection.