Using styrofoam is the cheapest way to insulate. You can buy it in 4x8 sheets of various thickness and cut to fit.
On my home, I used 8" thick fiberglass insulation. It came encased in plastic. I installed mine before I installed my new skirting, but the concept is still the same.
First I put down some narrow strips of styrofoam on the ground. I used long nails to keep them in place. Most of my styrofoam was my old thermo skirting cut into strips.
Then I took the insulation and nailed it to a 2x2 that was added at the top. I used roofing nail gun to make quick work. Before I nailed, I bunched up the plastic wrap so it won't tear out.
Below are a couple pics.
Ever since I have added the 8" fiberglass insulation, underneath my home has never dropped below freezing, and I live in artic South Dakota. During the winter my floors are warmer and I believe my heat bill is less. Since it doesn't freeze underneath my home, I no longer have frost heave issues. When you do insulate, you still need vents although I only opened mine during the warmer months.