Note that the key to retaining the symetry is to bend only one side and measure. Then when it's just right to bend the other and measure again using the first bent side as your new reference.
FB mentioned this but I thought it worth stressing as if you don't do it with due care and consideration and planning you can end up with a non symetrical frame.
Oh, and Sheldon describes doing the job with a long 2x4 as a lever. It may well be worth using such a trick as it give you a more delicate feel of the yeild point and you're not popping a blood vessel trying to get enough force to do the bending.