Both of these methods are bulky, and somewhat effort intensive, but I haven't seen anything better.
1) Part 1:
http://www.silentpcreview.com/module...icles&secid=14
Part 2:
http://www.silentpcreview.com/module...rtid=11&page=1
2)
http://www.silentpcreview.com/module...rtid=67&page=1
Some off the shelf ones exist.
Silent Systems makes a good insulated metal box that reduces noise significantly, though you need a low flow fan blowing over it; requires one 5 1/4" bay. Don't remember where to get them from.
Innovatek makes a vibration dampener that works well, and requires only one 5 1/4" bay. From
http://www.sharkacorp.com among others
And this one works, and looks bad ass, but it's pricey
http://www.silentpcreview.com/module...rtid=23&page=1
Dunno if that helps, hopefully something there will give you an idea.
-j