What can I do to prevent the concerta crash?

Short acting med. Using a shorter acting, lower dose of methylphenidate ( the active ingredient in concerta) taken in the afternoon might help.
Squeecking dose. Concerta (long acting version of methylphenidate)can last between 8 to 10 hours, however the tapering of effective dose starts at 6 hours. Try taking a short acting methylphenidate about that time. Your physician would the best judge.