use a proper cassette tool which has an integrated handle and a pin that goes through the axle, and a proper, good, chain whip, not an oil filter wrench, or a crap chain whip.
e.g.
http://www.chainreactioncycles.com/Models.aspx?ModelID=16389
but the best tip is to kneel behind the wheel, and put the tools on the other side of it to you, in the 20-past-4 position. then lean over the wheel with your hands on the tool handles and put your weight on them. If you are using good, proper, tools then the force of all your upper body weight will translate into torque removing the casssette.
The little bit of chain on the filter whip that is attached both ends goes on the underside of the cassette, and the long bit with one end attached wraps round the top. get this in place first, at the right angle, then put the cassette tool on at the right orientation relative to thechain whip.
credit to tim benet for that tip. i do it like that and it always works no matter how tight. in fact i think his avatar is of him nearly in the position in which to do it