Here's some advice from Brian Cheney on speaker stands (copied from their European site):

How to improve your stand

1. Make sure it is the right height. Usual stand height of 50cm may be OK, but might be taller or shorter for you, depending on your height and/or your seat height.

2. Add weight. A steel sheet on the stand on the speaker, weights on top of the speaker, sand in the stand frame.

3. Add a baffle. A "beard" installed on the front of the stand, top to floor, vastly improves the sound. Almost any material will work, even cardboard. It's puzzling that manufacturers turn out open frame stands at all. The cavity below the woofer destroys bass response, as Paul Klipsch pointed out 35 years ago. Adding a front baffle the same width as the speaker and extending from the top of the stand to the floor gives the bass a boundary to travel along and increases efficiency below 100Hz up to 6 dB.

http://www.vmpseurope.com/aboutVMPS.htm