I use a Dexter hand cutter I picked up on ebay for $5 and a heavy corked back ruler I found on Light Impressions. After a few failures I am making perfect mats. I majored in advertising design in college (way before photoshop days) so I am used to measuring and trying to be exact. The old adage measure twice and cut once applies. I also find if I lose concentration I screw up
As others have said changing blades often is important. I get two 11X14 mats cut (8 cuts total) from each blade. Just to see I've tried a third mat from the same blade and by the last cut the edge was not clean.