Also, I don't know if anyone else stated, but I will let you in on a little secret. When using a rotary cutter and a longer ruler than your hand, stop the blade at the end of your hand and reposition your hand further down the ruler. Again, stop the blade before moving your hand. When you go past your hand it acts like a pivot point and you will have less than straight lines. Another trick I learned is to rest the outer two fingers on your fabric and the other three on the ruler.
I haven't tried it it, but Joanns sells a product that you can mount on the bottom of the ruler to aid in the ruler from not slipping. I was in a shop hop and the lady that was cutting my fabric show me the hand trick and then let me try. It diffinitly will help as it did me. I heard that the sell little stick on circles that grip the fabric.