I have recently 'discovered' these. The sort I got DO NOT need a vacuum to use - you just roll them up by hand and they expel the air through a special flat valve as you go and when you flatten them out the air stays expelled. Saves about 80% in space terms. Great for packing spare shirt, socks, jocks etc in your carry-on.
As an example:
http://www.amazon.co.uk/Vacuum-Pack-...ef=pd_sim_kh_1
I guess others here use them and have done so for some time - haven't they?