Verb tense shifts are probably the most common and troublesome mistakes we see in student creative writing. Below are links and exercises to help you identify and eliminate these errors.
Start Here: A Power Point Overview of Verb Tense Shifting with Interactive Exercises
If you need more in-depth help: Purdue University's Complete Review of Tense Shifting
Practice: Purdue University's Verb Tense Shifting Exercises
Practice: Longman's Verb Tense Review Exercises
Review and practice some more: Review of Verb Tenses with Built-In Review Exercises
Brush up one last time to be sure: A Short, Simple Review of Tense Shifting