Riding Lawn Mowers

I just finished installing the third new drive belt on my YTH2348 mower and it already looks too loose. The reason for replacing the belts is that they begin to slip and I lose motion, particularly in reverse. I seem to only get about one season out of a new belt. Is there some way I can tighten this belt up. I also replaced one of the idler v-pulleys that was worn, thinking that might help tighten the belt up, but it still seems awfully loose to me. I can get motion now, but I'm afraid this belt won't last long either. It's like I need a bigger diameter clutch pulley or something.