For longer cuts you might also want to look at one of the O'Lipfa rulers. tried to find a link with one in action, but couldn't. Basically, there is a iip on the bottom of the ruler which you let it lap over the edge of the cutting mat. This keeps the ruler straight and secure as you cut.
With any ruler, you need to add something to the bottom to keep it from sliding. There are many products on the market for this purpose.
http://www.amazon.com/OLipfa-18-Inch...lip+edge+ruler