BIG TRICK LEARNED DIRECTLY FROM CAROL DOAK:
Take a piece of strapping tape and tape the card you use to fold on the line to the ruler. Line it up where it's supposed to be and tape it to one end. When you're paper piecing, put the card along the line, fold, bring the ruler over, cut. VERY VERY EASY!
If you use a piece of template plastic for your card it never frays or becomes soft on the edge.
If you invest in an Add-a-Quarter ruler (or Add-an-Eighth for tiny projects) it really will help. That's a ruler with a "lip" on it that's specifically designed for paper piecing.