I look for something that is:

1) lightweight
2) porous/well-draining
3) Cheap!

So, when I did bananas, I try to remove a good bit of soil from the roots, trim back the longest roots, fit the plant into the smallest pot possible, and fill in with 1/3 Miracle Grow potting soil and 2/3 with an inexpensive soil additive called Claybreaker. It's mostly pine bark and gypsum. It's used to break up clay soils. It's pretty cheap at Home Depot and goes a long way. It looks a lot like mulch.

...and by the way, I only trim the pseudostems down to about 3 feet. I keep them in my garage. They are usually starting to put out leaves again by April.