My vote is for sand, preferably in a water proof bag. i also like to flip them about once a week just to make sure they haven't frozen solid. i think its been said in another thread, but i'll say it again, as long as you have the weight in the med it doesn't really matter where it is in the bed. some like it as far to the rear as possible, some like it directly over the axle, the safest place is up against the cab. not only does it have no where to go in the event of a wreck, it also adds weight to the front to help keep the truck pointed the direction you want it.