I recently rescued a young border collie/chow mix and have been taking him to the dog parks every other day along with my other 2 dogs. He was most likely kept in a crate most of his life because he doesn't know how to walk on a leash nor socialize with other dogs. At first he just barked at the other dogs entering the park, but a few weeks ago he started to wait at the gate and literally pounced on dogs entering the park and nip at their ears, causing a complete ruckus! I stopped this aggressive behavior by carrying a spray bottle filled with water set to a thin but powerful stream (like a water gun)... every time I see that obsessed look as a dog is approaching the gate from the parking lot I say "NO, back off" and then spray him water on his face (doesn't hurt, just shocks him). It has also been very helpful when other dogs get into a fight. A few squirts and the dogs break apart. I highly suggest people carry a spray bottle or a water gun with them at the dog park. Just be careful you don't spray other dogs with it unless it's a bad fight you are breaking up -- the owners might get pissed off!