Vac seal (if you have enough cash to have to bag it up and stash it, you can splurge on a vacuum sealer) the money up. Take a cooler and put a one inch barrier of spray foam insulation on the bottom, then put your bags/bricks of money in the cooler; leaving a one inch gap on each side. Spray foam that in, put the cover on, Gorilla tape that thing closed, throw her in a garbage bag, and bury it below the frost line. Lasted one guy I forget how long, but long enough to do a bump and come back out to grab his dough.
Basic stuff here; I heard that tidbit on The Discovery Channel. My only suggestion, write down the geographic coordinates! You don't want to have $20k lost in a field somewhere.
Edit: just reread this. If you are putting it in your house, my vote is under the matress; bare Benjamins.