...only get more worked up and associate this with other dogs.  He will immediately start getting upset when he sees another dog anticipating your reaction.

If you have a large dog which you feel may attack another dog while on the lead, use a halter to control him.
 

Aggression towards Visitors

When visitors call, shut your dog in another room until you have achieved control.

Arrange for someone to visit that you know will not be frightened of the dog and will follow your instructions to assist with training. The first step is to tell this person not to respond to the dog by either yelling at it or making a fuss, either one will get the dog excited which is exactly the opposite reaction you want.

In the first session put the dog out of the room and bring him in when the 'visitor' has sat down. Talk quietly for a few minutes ignoring the dog then end the visit. Repeat this process a few times until the 'visitor' can be met at the door. It is important that the dog is taught to sit beside you at the door when someone calls, teach him to do this and not bark and jump at the door or rush forward as the visitor walks.

Until you have gained this control each time you get a real visitor put the dog out so that he is only learning from the situation you control.

Possessive Aggression

Remove the object the dog is possessive about whether it be a chair, a toy or whatever he will not let you touch while he is there.
Attain dominance over your dog by making him wait for food, attention etc. When you are dominant reintroduce the possessive item, as soon as the dog goes to it, tell him to go and lie down. If he becomes possessive again you have reintroduced the object to quickly.