[mesmark]

I generally have students practice over and over again with the third person singular by itself, no mixing - no does questions, no negatives and I usually use one verb at a time starting with likes, then has, plays, works, wants ... Total separate, I work with negatives and questions with does in the same fashion. But that's with children who don't mind repetition and slower paced progress.

For adults... I got nothing.

My adult students know the rules, practice just fine but can't use them when speaking. To quote you from long ago, maybe it doesn't really matter. I find that the third person singular -s or plural -s are one of the last things to get worked out, just like articles. The third person singular -s doesn't impede meaning, so unless they're not able to answer correctly for TOEIC, TOEFL, or other tests, I wouldn't worry too much about it.