For a quick good time, blankets...

A small flock of sheep also works well, and I found out once (long story) that an unconscious group of fur covered flight attendants absorbs reflections…a large puddle of old soup with a thick mold, burning tires (not indoors please), a sink hole between the speakers has to at the proper depth but is great. I once invited a group of shivering people at a bus stop into a sound check to simulate a crowd, but had to toss 'em out before the show…gotta have a ticket...

When using sheep I've found it best to stay clear of the Merino, English Leicester, Shetland and Awassi breed as they tend to seriously over damp the sound, while the Lleyn, Charollais, Wiltshire, Shropshire and Hampshire tend offer the best compromise in absorption. 

My only caveat is that on rainy days it gets a bit gamey and tends to ruin the whole affair.

All the best,
Nonoise

Make sure you wear your sheep boots!!

Bravo Erik.It's interesting the humorous (or is it humorless) comments made here, when in fact, you've provided a completely cost free method for "all" to learn how to improve the sound of their system regardless of the degree of refinement that have worked towards. With quiet time spent doing what you suggest massive improvements in clarity,detail,nuance,dynamics,soundstage and consequently musicality can be obtained.
Is it possible that those who makes jokes of your valuable (but cost free) insights have a vested financial interest?