Bev-these are all great tips. I have this same issue and I discovered that part of my problem was the angle at which I was holding my rotary cutter. If I hold the cutter lower and more parallel to the cutting mat then I was more likely to "shove my ruler" to the side and get a crooked cut. I now hold my rotary cutter higher-more upright, then I get a straight cut. You are only pressing down over the blade and not pushing the blade along. It really helped me. I have arthritis really bad in my thumbs, so I cannot spend a long time cutting at any stretch-I like for every cut to count.