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