I dont usually put any weight in mine. If I do have to I am going to use bags of salt that I can then use in my water softener>

 

Centering additional weight directly over rear axle is better than having the weight hang on the tail end.

 

I've got 3 five gallon buckets filled with sand I put in mine, I made a "compartment" for them to sit in so that don't slide around in the bed. I did put mine over the axle. The added weight seems to help most on the icy roads we get here in Michigan!