I had that problem until I started putting sandpaper dots on the under side of my ruler. A dot of rubber cement in several places (let it dry good) works also and is less expensive. Then double check that you are using the correct measurement at both ends of the ruler. Put your hand on the end of the ruler closes to you. Cut as far as your finger tips and pause. Move your hand up and cut again as far as your fingertips. Keep going until you've cut all the way across. It seems slow, but you'll get faster and more accurate as you get used to it.