The technique was easy as pie: Put stickers on the cars, and give them to the kids to play with. In this case, I labeled the cars by 10s. I gave the kids a jumbled pile and asked them to sort them out in order. Then I'd take a car or two out of the line-up and ask them what was missing. Finally, I'd pull out a car or two and ask them what car would come before or after if they were counting by 10s.
They caught on very quickly, and not surprisingly, were soon playing these simple games on their own. When they're ready, I'm planning to do the same for counting by 5s, this time using the plastic counting bears.