Achieving a continuous, smoothly varying curvature can be remarkably difficult for some of us. Well, maybe just for me and one or two others. Have found that simply holding a sheet of paper or something similar behind the workpiece helps me see the curvature better and make it smoother. Also holding one of several french curves (drafting templates) next to the piece helps show where the change in curvature is not as smooth as it might be.
Pieces shown (box elder and mesquite) are good practice and do not require much material.