okay buy one of those 45 degree triangles at staples or where ever, i think they come in a pack and are in the drafting section. you'll get a 45 degree angle evertime and you can cut off of them with your exacto.
the other way i do it is line up my strips of paper ( i used strpes how ever thick you have ... its one of the ways i extend the life of my paper sheets) then take the cross stripe and place it ontop of the other strip (like where it will go when im finished) draw a dot at the edge of the paper. then take a ruler and line up the corner of the strip to the dot i made and cut that line. only cut the one side then glue the miterd corner to the stright corner. works every time, im not sure if it's a perfect 45 corner but it looks darn close to it and depending on how thick you make your mats you cant always see the miter.
oh i usually make the mat frame a bit thicker than i want it to show up from behind the picture so its more like a mat with out the beef of paper.