Figure out why your dog is barking.  Here are some common reasons why:

Boredom Barking

If you find when you let your dog outside he barks, do not leave the dog out side alone.  Accompany him out side and keep him on a leash.  If the dog begins to bark, tell him sharply, "NAAH, NO BARK!" or "QUIET!"  When he quiets, quickly praise him.  This must be done quickly and sharply.

Never allow your dog to have lone periods outside alone.  If he is an outside dog, work on making him an inside dog.

If there is no option than to leave him outside, you may consider a no-bark training collar.  This is a collar designed not to shock the dog, but to train him to be quiet.  This collar will allow a dog to bark a time or two, but when used correctly will teach him not to bark without painful shock.  Contact the instructor from the store where you purchased the collar for proper use.

If at all possible, however, attempt to bring your dog in the house.  If it is untrustworthy, crate training is a simple solution.

You can not correct your dog for barking after the fact.

It only takes a short time for a dog to train its owner.  If only one time the dog barks for an item and receives it, you have set a behavior in motion.

Example:  removing your dog from a crate while barking.

A dog will attempt many different ways of getting what they want.  Again, do not give the dog what they want until you get what you want.  If you do not want a dog to bark to come out of the crate, only remove him when he is quiet.  If your dog barks for food, do not give him food until he is quiet.  Reward your dog for behavior you want repeated.