Why not have multiple TOTM winners each month?
 2WD TOTM and 4WD TOTM in the same month.
 Multiple year or generation TOTM winners in a single month.
 TOTM by age demographic (possibly consider multiple winners in a single month, 18 to 30 year old class, 31 to 50 year old class, 50+ class )
 TOTM by work demographic (White Collar TOTM, Blue Collar TOTM, etc.).
Multiple class winners opens the contest to more members and creates more interest. I think 2 or 3 classes per month would keep the interest of members without wearing everybody out with the voting process.
Contests could all run at the same time or at various times throughout the month. All members are eligible to vote in all categories.
It doesn't cost anything to have multiple contests and expands the base of interest.
I am not a fan of the minimum post requirement. I have been a member since April of 2010 and only have 50 posts. I read more than I post, but I should have member "cred" based on my length of membership, not my number of posts.