I also use timelines visually reinforce the concept because that´s what helped me finally understand it. Although, also based on my experience, I think the real key to mastering preterit/imperfect is through authentic communication. Well, that´s true for most everything in language learning actually!
¡Y ahora al cortometraje! The vivid imagery is perfect to inspire students to write descriptions using the imperfect, and although only 3 minutes, there are plenty of events to list to practice the preterit. Once students have time to brainstorm their own ideas, we go over them together. I draw a timeline on the board and mark the events as specific points. As students describe the settings/characters, I make squiggly lines in different colors along the timeline. Finally, students combine the descriptions with the narration of events to write a short story based on the short film.
I also recommend this video by the Travel Channel.