One thing I read somewhere is to use the same brand of blades as what your mat is. I know when I use my Fiskars blades on an Omnigrid mat they dull really fast. So now I have different cutters that I use at home and at my sewing group. The Olfa blades don't seem to dull very fast on the Omnigrid mat.
I got a Martelli cutter and blade in May at a quilt show and it's not showing any signs of dulling yet. They said their blades should last at least a year! We'll see.