Well, it depends on the extent of the damage. Try bending the frame rails with a steel tube for extra leverage. If the damage is too extensive, you might be better cutting the damaged part and welding new frame rails in.
If it's a metal frame you can try hammering it out if it is bent in a small amount. If it is bent sharply your best bet may be to replace that portion of the frame and weld in a new piece. A serious bend or dent to the frame can cause a weak spot that may break with use.