Past tense pronnciation? Do you mean they can't remember irregular verb tenses? Go, goed, went? That kind of thing? That's just rote memorization and practice.

Otherwise, I enjoy, as do my students, tongue twisters for pronunciation. You can use the old standards, or just make up sentences with the problem sounds repeating in them. I usually start a class with one, letting them practice a few times, and try to work into the lesson the problem words at least a couple of times.

Where are you in the world? And what are the pronunciation problem sounds and words? A definitive answer depends on to whom you are teaching.