What can I do about seams that won't lie flat?

If a seam is stretching or not sewing smoothly, try sewing into the seam "Seams Great" by Dritz. "Seams Great" is a thin bias tricot which may be used to finish seam or hem edges as well as being a great stabilizer. "Seam Tape" by Dritz is another helpful product. It is a thin tricot that doesn't stretch, but is straight grain.

I keep these products in my sewing box, as I never know when I will encounter a problem.