I used to have to cut mats by hand for art shows (for pastels and such). I used a
Dexter Mat cutter. It might be a weird choice for a photog, but I started in art, so that's why I use it.
When I bought it 10 years ago it was $20. Its probably around $30 now. You can get one at an art supply store. Its a silver pear-shaped thing that holds a razor blade that is specially made for it, at a 45 degree angle to get the beveled edge. To cut straight lines, y ou use a T-Square or thick ruler and push down with one hand while you drag the Dexter with the other. Then nip the corners with a reg. exacto-knife and pop out the center. Its not hard and after a little practice, it looks prefab or better.
I'm sure that was as clear as mud...but it works good and unless you are doing bulk - it works well. IF you are doing bulk and dont want to buy a big thing - go to Michaels. In the matting department they will cut any mat you need for about $10.