Establish some rules:
*) If, when you try to enter the park, the dogs already in the park charge up to the gate to check out your dog don't go in. Wait between the entrance gates until the dogs in the park lose interest in your dog and walk away from the gate.
*) Walk into the park heading away from the group of dogs so your dog can enter the group more naturally and avoid blasting into the middle of a play group.
*) Release your dog from his leash as soon as you get him heading away from the other dogs.
*) If there are too many dogs coming and going at that park to allow you to do this find a less busy park or go there at off-peak times.
*) Play should be interrupted every 3-5 minutes. The best way to interrupt play is to wait until the play comes to natural conclusion and the dogs shake off. Immediately thereafter, the playmates should be called to their owners' sides and restricted until everyone calms down; 3 - 5 minutes is usually sufficient.
*) Go home if a fight breaks out, regardless of whether or not your dog was involved, or even if the fight was not particularly near you. That kind of aroused energy hangs in the air and effects all the dogs.
*) Different breeds have different play styles, some rougher than others. Nevertheless, go home if the play gets too wild.
*) Behaviors to look for are: dogs frequently up on their hind legs; grabbing, dragging or shaking another dog, especially by the throat, scruff of the neck, ears, lips, feet or legs; one dog gets ganged up on by the group; one dog/your dog is always on the bottom.
*) Dogs should not be allowed to play while wearing prong or choke collars.
*) Leave immediately if dogs with known behavioral problems show up.
You cannot police the dog park. Do not try to enforce rules with the other owners. Just go home. Keep you and your dog safe.