the comfiest position i found when pushing was on the bed on my knees propped up with a beanbag. this felt the most natural for the position that DD was taking on her way down. it all fell to craziness with the OB 'needing' me to push on my back. i negotiated (somehow!) to have the bed back raised and the end of the bed fully dropped so that i was in as much of a sitting position as possible. but it still was ALOT more hard work than it should of been.

IMO go to a birth centre! that way you can be pretty much free to birth in as many positions as you feel comfortable with. otherwise, i reckon ask your care provider what the reality of the pushing stage is (in terms of how they want you to be positioned), and try to negotiate this in your birth plan (and make sure DH or any other support person is aware of this too).

HTH