Lay the bar on a concrete floor or some heavy flat steel, with the bow up,find the highest area in the bow and hammer it there with a 2 or 3lb. hammer until its straight, don't hit near the bar rails.
The first thing is to find out exactly where the center of the bend is and whether there is any twist as well. (whether the bend is 90 deg to the bar or slightly diagonal); a bar with twist used on big wood gives some strange results. Usually you have to straighten beyond then back to keep the bend from returning. A bit of hammering while the bar is bent back beyond straight helps set the metal.The caution about hammering near the hardened bar rails is good advice.
I straightened a 16" husky bar once by putting in a press resting on blocks like lakeside mentioned and pressed down on the top of the bowed area. It really surprised me at how far past center I had to bend it to get it to be straightened. if I didn't take it far enough past center than it would still retain some bow in it.