For a quick good time, blankets...

erik, if you have the speakers on hardwood or tile floors, any kind of sound absorption in front and between them on the floor is an absolute requirement. Also, depending on the speaker type, some absorption (or defraction) on the ceilings between/front the speakers will improve the SQ. This is all about removing unwanted floor/ceiling reflections and having them mixed with the direct sound from the speakers as they reach our ears at different times, etc. But you probably knew all this already.

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!!