Tips for keeping mice out?

I've had a long standing battle with mice in my trailer and have tried just about everything. The trailer is used as a 'cabin' so it is permanently located in the semi-wilderness of northwest Arizona (high desert). I recently purchased and applied the "Mouse-Free" product, and am now waiting to see if it works. I have started a long term product evaluation here:

http://springvalleyranches.com/index.php/more-stuff/reviews/81-mouse-free

I have been exchanging emails with the owner (?) of the Mouse-Free company, and he seems very confident his product will solve my problem. Wish me luck!