Don't have chalk or a washable fabric pen handy and want to maintain a straight line when sewing?
Here's a good rule of thumb: Draw two evenly spaced parallel lines with a pen or Sharpie marker on your leading thumb.
As you sew, keep the top mark that you drew on your thumb lined up with the edge of the fabric. The bottom mark on your thumb will be where your "imaginary" line will be. Viola! No more wavy stitch marks!