I've also found that a new x-acto blade works best for me.
I also use a very hard surface to make the cuts on. I found that if I used my cutting mat, I would end up with a bend in the part. I have been using a scrap piece of glass to make cuts on. (Not sure how to describe the glass, but I picked it up from a fellow in our modeling club. It is about 3" x 5" and about 1/2" thick - it was made for some purpose, as the edges are rounded) Since I have been cutting on the glass the only thing I need to worry about is having the parts "fly" after being cut. If I use a small piece of tape - that solves that issue too!