Folding your fabric correctly is the first key to getting straight cuts. Make sure there is no 'bubble' at the fold and then smooth and press. Put sandpaper or rubber dots on the corner of your ruler. Or, get a couple of spring clamps to hold your ruler in place - or all of those! Another tool you might want to check is the Quilt Cut. It's a bit spendy but holds your fabric tightly clamped in place, all the rulers are an integral part of the unit so they don't slip at all. Can do straight or just about any angle cut you might need.