I only use my 24 inch ruler when I must, like trimming off a quilt, or cutting long strips of binding. My favorite ruler of all is the June Tailor Shape Cut ruler that has laser cut slots allowing you to cut quickly and accurately. This ruler doesn't slip. Here is a link to it:

June Tailor Shape Cut Plus at Joann.com

June Tailor also makes a Half/Quarter Square Triangle ruler that gives you perfect HST up to 6.5 inches every time. This ruler doesn't slip.

I have also fallen in love with Creative Grids rulers, especially their specialty rulers, like the Storm at Sea. I recently bought several and have yet to use them. They have a grip on them that keeps them from slipping.