are you using proper axle nuts? if not, then order some track nuts with the integrated washer. you can get them pretty cheap at businesscycles.com or harriscyclery.com then, make sure you've properly tightened them. i usually tighten mine down to around 300 inch pounds. i rarely have a problem with losing chain tension. it seems like most of the folks i know that have lost chains when skidding or when the rear wheel is under heavy load was because their axle nuts were not tight enough.