Although I've been taking Laddie to dog parks lately primarily for socialization, turns out it's also a good opportunity for proofing line manners.
"Premac" is a type of positive reinforcement, in which a behavior being trained is rewarded by the opportunity to perform a different behavior the subject has a strong desire to do.
At the dog park, Laddie has a strong desire at times to sniff the grounds or interact with other dogs.
So periodically I call "heel". As soon as he complies, I reinforce with the release cue "go play". As a variation, I might have him retrieve a thrown or planted bumper or tennis ball first.
Compliance of course must be non-optional.