This excellent, free grammar practice activity site has wonderful practice activities for understanding verb tense shifts and many, many other grammar topics. And the computer scores the activities for you. This site is a great go-to not only for verb shift practice, but for lots of common grammar challenges.
This PowerPoint lesson is a great go-to for explaining problems with verb tense shifts to students who could care less about grammar. Chomp-Chomp uses hilarious graphics and sample sentences that almost "trick" students into learning.
This video is made to help high schoolers prepare for the SAT, but it is still a great walk through on verb tense for middle schoolers. The narrator talks through his thought process for discovering and working through tense problems in writing. Students often struggle to build the mental processes for finding and fixing tense problems, so this "think aloud" gives them a glimpse into the mind of a skilled reviser.
The teacher in this video doesn't have the most engaging voice, but she does a great job walking you through revision of a real piece of writing. I show this video to my class to show them how to examine a piece of writing for tense, and then ask them to grab pieces of their own writing and go to town with a colored pen, revising their verb tense. It's amazing for them to see how often they shift tense when they actually revise in this way.
Practice makes perfect, right? IXL's interactive sentence revision activities help students learn to mine their writing for errors. This is a great site to have students work through practice activities because not only does it score the activities for you, it also re-teaches the concept each time the students answers a question incorrectly.