Is it dry? In your situation, I'm usually getting it close to bow dimensions and then heating it. In my case I prefer using the stove burner and a little mineral oil on it till it gets hot enough to bend, and I will straighten it over my knee and go a bit past where straight it..Sometimes clamp it, but usually just hold it and then cool it off and let it sit for a day.
Other methods perhaps more popular here include a heat gun, and a form and you can heat it up and tighten the clamp as you go. And this works real good as well..
I just have this wild

6 year old so the thought of having a heat gun around make me a touch nervous at this point...
Rich-