Boxergirl always has the BEST advice!
I'll just add a few more thoughts...
Make him "heel" in the parking lot at the dog park... stop short if he tries to pull away from you to get to the "playground"... he must "give up the park" to "get the park"... (Doggie Zen).
When you go in the dog park, have a "check-in" command and use it a LOT. Start first in the parking lot, when he's still on leash and use LOTS of praise and excitement for a successful "check-in".
Having a hand signal for the "check-in" command is a HUGE help.
Denna knows "right here" as being "nose-to-palm-of-hand"... so, at the park when we first get there and she's right next to me (often still on leash), I'll say "Denna, Right Here" and hold out my palm... she does a nose touch, and I'll then say "GOOD!" - unfasten leash tell her to "Go Play!!"... and send her off to run around.
I'll follow her... and when she's looks back to see if I'm there, I'll say "Right Here!!" - palm out - she comes running for a nose touch and gets LOTS of praise!!!! (and another send off to "go play!"). If she doesn't look back, I follow and wait until she's taking a breather (so I know I have a chance of getting a good response), I'll get closer and again "Denna, Right Here!"... again, nose touch and "GOOD! Go Play!!"
If she ignores me and runs off, I do NOT yell after here... I follow... and when she's distracted and I can latch on her leash, I do... without saying a word... and we leave the park.
The gist is that as long as she checks in with the "right here" command, she gets to keep playing... as soon I get ignored, we leave.
After a while the "Right Here" command becomes a positive "I get to play more" association... so on the rare occasion you need to use it in an emergency, there's no question the dog will run back to you. And even if there's no play after "capture"... there's ALWAYS lots of love and praise for a successful return/recall/capture event.
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As for going back to basics... Denna is pushing 7 years, and WE need some refreshers, too... she now has a bum knee (torn CCL), which has limited our outings and her social skills have suffered. I'm thinking of enrolling in another obedience class (or nose work, or some other fun activity) to force us to get together with other like minded dogs in a structured setting again... It's never too late to go back to class!