My best results for even heating have been with a heat gun or large hair dryer. It's important to work both sides and be patient so as to not overheat the surface. Also wait for it to just reach its yield point. Don't try to force it and don't let it heat much beyond its yield point or the curve is less likely to be even.
Originally Posted by yellowcat
It's best to not directly heat the entire area to be curved, as the heat will diffuse laterally. Heating the entire radius will mean that the areas you want to keep unbent will be harder to control.
If at all possible practice first on a piece large enough to get a feel for it.