You can start out by checking to make sure your rulers are as "true" as they can be.
I've had two rulers that I can remember that weren't even true to themselves.
By that I mean - if you are using a square, does it measure the same in both directions.
Start out by making sure your pattern/templates "fit" together correctly. Usually they do, but not always.
Do the lines on your mat line up with the lines on your rulers?
I have a June Tailor Cut N Press - which I like a lot - but the lines are not accurate - I just use the lines to get my corners square and edges straight. They may have been accurate at the time of manufacture, but mine isn't anymore.
I have better success cutting the long strips parallel to the selvage instead of across - that way I have a least two edges with a straight grain line. When I try cutting across the width, I end up either with a bias edge because of the way the fabric is processed or a bend in the middle.
Make sure your blade is sharp.