The standard method of treatment for this problem can be corrected in a three step process.

Step One

As your dog walks toward another dog, at the moment the aggression is displayed, correct the dog with a leash-check or other form of negative reinforcement at the exact moment the dog displays aggression. Timing here is important because a correction after the aggression begins may cause further stimulation. Say "Leave-It!" and continue walking.

Step Two

As your dog walks toward another dog and looks at him, correct as above (now we are correcting him for even thinking about it.)

Step Three

As your dog walks toward another dog and looks away from him, praise him and give a food treat.

Using positive and negative reinforcement can often times act to re-shape this behavior. However, if your dog is older and extremely aggressive with other dogs, you should consult with a qualified animal behaviorist before undertaking any behavior modification.