I use a "O'LIPFA Lip Edge Ruler" that I've had for years. But I did have trouble when cutting multiple layers that the ruler would not stay put on the edge of my cutting mat. To solve this problem I glued a carpenter's pencil (can be found at any home improvement store) on the lip. In case you're not familiar with the pencil - it's flat and does not have an eraser or any metal and can be cut easily to any size.