I cannot believe that the reefer goddess, good ol' Stinky, didn't mention this one (maybe you do know, but I still feel special :-p).

Spray the inside of your containers (before planting, lol) with a product called Griffin's Spin-Out. It contains copper hydroxide, which keep the roots from growing into the side of the containers, they'll grow within an 1/8" of it. It works just like the FIM technique or topping. The roots are chemically "cut" when they reach the outside of the container. In return, you get twice the amounts of roots, and they will grow into the center of the pot.

This method doubles your root volume, but allows many more roots to grow in the center of the pot, instead of at the outside of the container. I would say that this method will give you another 4-6 months of clone time before you will need to replace it.

Ohh wait, Stinky said root pruning, which is what this is.... hahaha well there ya go Chemical root pruning, who would have thunk it?