Nancy's advice is great! Dogs that seem "aggressive" on leash are usually acting out because they are afraid. They are afraid of the new dogs/people/places, and they lash out because it's a warning "stay away from me!"
Yukon became VERY reactive (lunging/barking) at other dogs when on leash when he was a teenager. Yukon was attacked by a supposedly "friendly" dog during an on-leash greeting. Although we're still working on it, we have had unbelievable improvement using desensitization and the Control Unleashed program (particularly the Look At That game). He can look at the other dogs and turn back to us for a reward. *Also, we NEVER go up and greet strange dogs anymore, EVER.* A few weeks ago, we walked Yukon on leash by a dog park with lots of dogs playing and running around, and he didn't react at all! He knew that we had cookies for him, so he'd look at the dogs and back at us for his cookies! I had another major success with him shortly thereafter when picking him up from daycare. I was taking him for a potty walk before getting in the car, and another dog (a big, black dog -- the type of dog that he is most afraid of) started to come up to us with his owner (they were coming back to pick up their poo), so they were about 25 feet away. The dog started to bark at Yukon, and Yukon looked at the dog. I didn't have a treat, but Yukon looked at me, and we just turned around and walked away. Yukon never reacted at all! I think it is because he has learned the new rule structure --- he never has to go up to another strange dog. I will protect him, he doesn't have to scare the other dog away.
I just posted this in response to Paula's question about Sweet Pea, but I'll post it here too. It is a really great book with a step-by-step program for how to deal with this type of situation. Control Unleashed by Leslie McDevitt:
http://www.cleanrun.com/index.cfm?fuseaction=product.display&Product_ID=1328There's a Games Demonstration DVD (you can also buy a Video On Demand of individual games):
http://www.cleanrun.com/index.cfm?fuseaction=product.display&Product_ID=2357And an awesome, very active Yahoo group for folks who have read the book and are working through the program.
Good luck!