Use the "Sit" Cue to keep your dog next to you while you check out at the vet or pet store, as a polite way to greet instead of jumping, to prevent running out the door when opened, and any situation where you need to them to be still for a moment.
Hold a treat, palm up, to your dog's lips.
Say the cue "Sit" ONE TIME.
Slowly bring the treat up over their head and toward their rump so that they look straight up at the treat.
As they look up at the treat, their bottom should tuck under into a Sit. When they get into the Sit position, give the Reward Earned cue "YES!!" and then give them the treat.
After 5 repetitions, leave the treat in your pouch. Put your empty fingers to their lips and repeat steps 2-4, getting the treat from your pouch AFTER you give the Reward Earned cue.
After practicing a few times with your dog facing you, practice the Sit with them beside you. Sit next to you is more useful than in front when you're out in public with your dog.
Practice on-leash in different rooms inside your home this week. Next week, do it outside with more distractions.