I use 6x24 ruler for just about everything . It is fiskars I believe and the marked side slips less. The non marked side is like a skating rink. If I have to cut a long strip, I place my hand in the middle of the ruler with the ball of my hand, thumb, and index and middle finger applying the pressure to the ruler. It helps if you do not push your rotary cutter too much against the ruler but rather glide alongside it. If you put too much pressure with your cutter along the side of the ruler either top or bottom side will slip. It takes practice, absolutely, and this is what works for me. When I get tired or rushed, forget it. All my cuts are wonky. Keep at it, take a deep breath, sip of wine and enjoy. You'll be a pro in no time. Happy quilting.