you're going to get a dozen answers from 10 people. and you're going to get more advice than you asked for as well regarding placement of monitors on the desk verses on stands and how far away from the wall they should be and how far from you they should be and how far apart they should be and much much more than you ever wanted to know.
to answer your question directly without overloading you with information you didn't ask, I would suggest using these:
http://www.musiciansfriend.com/accessories/auralex-mopad-monitor-isolation-pads I understand they're not cheap, but they work pretty well for their intention.
Now: a little more info you didn't ask for:
I have my monitors set on these on stands, about 6 feet apart, about 3 feet from the back wall. this eliminates the problem of having your computer monitor in the way of sound and reflections, however, it adds a problem for editing because if you're at the keyboard and monitor, you're too close to the monitors.
so I always back up so that I'm in the sweet spot and don't do any "editing on the fly" but take notes, go back to the keyboard, edit, move my chair back and listen, repeat, rinse and repeat... :-)
the monitors should make an equilateral triangle with your head (from a bird's view). in other words, you should be the same distance from each speaker and that distance should be the same distance as each speaker is to the other.
they should be at the height where your ears are level with the bottom of the tweeters (assuming 2 way speakers oriented vertically). they can be pointed downward slightly if the tweeters are actually a little higher than your ears.
see? told you you'd get more info than you asked for!
