All of these are great tips....a sturdy table at the right height was a big one for me.
But what seemed to help me the most was to use the blank side of my cutting mat. That way, you are truly using a ruler to measure your fabric...not trying to line up sets of lines on the ruler with sets of lines on the mat. This method really has helped me become an accurate cutter. To square up the end of a folded piece of fabric, use a (preferably large) square rule butted up to your long cutting ruler. And squaring up the cut edge of the fabric is a big one for me, especially when cutting strips.
Of course, nothing helps like practice and lots of it!