I used a shock collar on my dogs for aggression, and found it was worthless.
What I did find that worked was if my dog showed ANY sign of aggression, I would make him think he was about to die. Scream, yell, grab his collar, etc. Will this work with your dog? Who knows. Personally, I really didn't like doing it, but I had hired a behaviorist because I was having serious problems.
You should also check out this site, as it has TONS of great articles, many of which helped me: