Use good quality rulers, not "chepos" that may be inaccurate. Use starch. Use a quilter"s rotary cutter and mat (not scrapbooking quality), use a firm table with no bounce, and use the ruler to measure, not the gridlines on the mat. If you are cutting wide widths (for a border for example) place 2 rulers next to each other. Cutting mat lines have a width to them and may be quite inaccurate. When cutting, place your baby finger or baby and ring fingers off the ruler and onto the fabric.
Now fold your fabric in half, so that the selvage touches the original fold. Your fabric will be approx. 10-11" deep by whatever length you chose. Cut half way up, and leave your blade IN THE FABRIC, then move your fingers up (toward the top third of the fabric) and continue cutting. I like to use a Creative Grids 2.5 x 12" ruler. When using a 24" ruler it can be hard to prevent slippage.
Read Harriet Hargrave's books called "University series" especially the Freshman Year volume as ideals with accuracy. Good luck!