While shopping for a new straight edge to replace my T-square I think I found something you'd like. Look into the Alumicutter. It has a rubber non-skid pad with a slight metal lip on either side, so when you press down on the ruler to steady it the metal lip sits flush on the paper. No slip and no gap!

Are you using a basic aluminum ruler, or a non-skid one? Rulers without non-skid backing will be a complete nightmare to do straight lines on. I invested in an Alumicutter and use it with a basic utility knife to get clean, straight lines.