Cheryl,
If you are using a longer (24") ruler, you need to cut for a short distance, 12 or so inches. Stop, reposition your hand by moving forward about 12", closer to the opposite end, and continue cutting.
Also, there are several "grippie" dots that can be stuck on the back side of your ruler which help hold in place while cutting. These dots are available to most sewing stores and most catalogs.
Hope this helps. Suzanne