We present PortraitSketch, an interactive drawing system that helps novices create pleasing, recognizable face sketches without requiring prior artistic training. As the user traces over a source portrait photograph, PortraitSketch automatically adjusts the geometry and stroke parameters (thickness, opacity, etc.) to improve the aesthetic quality of the sketch. We present algorithms for adjusting both outlines and shading strokes based on important features of the underlying source image. In contrast to automatic stylization systems, PortraitSketch is designed to encourage a sense of ownership and accomplishment in the user. To this end, all adjustments are performed in real-time, and the user ends up directly drawing all strokes on the canvas. The findings from our user study suggest that users prefer drawing with some automatic assistance, thereby producing better drawings, and that assistance does not decrease the perceived level of involvement in the creative process.