I rely on Acu-Arc adjustable curves for most of my layouts. Made of interlocking side-by-side layers of flexible butyrate plastic, they are easily shaped to the desired curve. Most importantly, unlike standard drafting flexible curves made of a metal core covered with plastic, Acu-Arcs easily and almost automatically take on a smooth, fair curve.
Shape the Acu-Arc, then hold it on the wood and use a pencil to trace only the section close to your hand to prevent pushing the curve out of shape. Carefully reposition your hand further down the curve, trace there, and so forth. Highly recommended, Acu-Arcs are available in lengths of 18″, 24″, 36″, 48″, and 72″.