I have had aggressive dogs that I have adopted before. Of course the first thing I want to do is teach them to sit. I use a verbal and a signal command. This at least gives me a chance to put the dog in a calmer state of mind if he gets too overly excited outside. One trick I learned is if he acts aggressive is I give a big hello to whatever he is being aggressive to. Lets say everytime he passes buddy the basset hounds yard and buddy barks he goes nuts, so I give a big happy hello in a cheerful voice while kind of wagging my body a little and I say, Hi Buddy, good boy, good to see you and I act like buddy is a long lost army pal. Generally the aggressive dog i am with calms down right away.( this works only if the dog is being aggressive out of fear or being in a strange place , not a dog that is aggressive due to abuse or brain damage: you would be surprised at how many people have given young pitbull pups cocaine). I have found if a dog thinks your are friendly towards something he is a bit leary of: he generally accepts your opinion on the matter.