Not that I am an expert, but here are a few things that have helped me.
-Mean what you say. (If you promise a spanking, deliver it asap. If you say they will be denied a priviledge, don't back down even a bit)
-As they say "Walk softly and carry a big stick". Keeping a wooden spatula or spoon in the van is a nice reminder for everyone. Everyone will know it is there for the just-in-case situation. Another at home in the same place all the time.
-Sometimes I still have to implement "one hand on the cart" with my son who is 8yrs old.
-No one may crawl on the floor, climb, advertise a product to me, beg, cry, whine, crawl through a clothesrack or peek under dressing room doors.
-Parking lots really scare me and I, personally, have threatened serious discipline for disobedience or straying there.
-Implement a 5 square rule. I learned this in retailing. In retail, if you the employee come within 10 linoleum squares of a customer, you are to greet them, etc. With your child, shorten that to 5. If they leave that 5 square perimeter... sudden discipline should follow.
-Keep a tally and deliver asap. 3 infractions for Jim, 2 infractions for Sally and they get a dose of trouble for each issue. If you swat- break the swats into the appropriate groups.(If you normally give 5 swats and they broke rule 3 times give notice that the next set is now beginning after each 5 swats for a grand total of 15) If they get time out- same concept of delivery.
-Quote a few key scriptures. They will get to know them well!!! Ask "Are you loving your neighbor?" "Is this showing honor to your mother?" "Are you delighting in another's wrong doing?" "Is that something a foolish man does or a wise man?"
-NEVER EVER reward bad behavior or bribe. Good behavior is expected. It is not a treat your child blesses you with.