In order of importance:

1) Remove the trigger. If there's nothing to trigger the dog, there's nothing to bark at. However, there are times when you cannot remove the trigger (i.e. the neighbor's barking dog is making your dog bark). In that case, move down to #4.
2) Exercise (I'm sure you already have this down)
3) Mental exercise
4) Never give the dog the opportunity to bark. If he barks when he's outside because everybody else is barking, bring him back inside. If he needs to go out for a potty, supervise him. As soon as he potties, bring him back inside.
5) Teach Quiet. Present the trigger to the dog, wait for him to stop (he will, eventually). As soon as he stops, say "Quiet" and treat. Repeat repeat repeat. Eventually, you can say "Quiet" in the middle of his barking and he will stop.

Also, to immediately stop the barking, present a deterrent every time he barks. I am *not* a fan of squirt bottles. Instead, I like using a can of pennies. Every time the dog barks, throw the bottle in the dog's direction, as soon as he stops barking, say "Quiet" and treat the dog.