Currently I skip players that don't do one of two things when it is their turn:
a) ask a question regarding the combat
b) declare what action they are taking and roll the dice
If they don't do it, I skip the player and move on in the initative order. The can interrupt me to get their action and keep their place in the initiative order if they declare what action they are taking and rolling the dice.
I try to say who's turn it is and who the next player will be. That way you should get a very short warning. You have to pay attention all the time or just revert to hitting the closest thing with the first at-will you see.
99% of the time this works out perfectly and a round goes in 5-15 minutes.
a) ask a question regarding the combat
b) declare what action they are taking and roll the dice
If they don't do it, I skip the player and move on in the initative order. The can interrupt me to get their action and keep their place in the initiative order if they declare what action they are taking and rolling the dice.

I try to say who's turn it is and who the next player will be. That way you should get a very short warning. You have to pay attention all the time or just revert to hitting the closest thing with the first at-will you see.
99% of the time this works out perfectly and a round goes in 5-15 minutes.