Personally, I like old carpet as an underlayment. It can be found for free, and its really thick.
I've also heard of having an overlayment, which protects your liner from landscaping rocks or animal claws putting holes through the liner. I'm not sure if the 20mil liner would work for this though.
I've also heard of having an overlayment, which protects your liner from landscaping rocks or animal claws putting holes through the liner. I'm not sure if the 20mil liner would work for this though.