Many good tips here!
I've seen many references to "the correct table height", but no definition of what that is, exactly.
The answer, of course, is different for everyone, since we are all different sizes. The best cutting height is at hip level. This way, you are able to bear down with proper force on both the ruler and the cutter.
Also, try cutting across your body, instead of away from you. Lay the fabric on your board with the fold on your left and the selveges on your right, lay the ruler crossways, and cut from right to left (or from left to right if you are a southpaw) on the far side of the ruler. This method utilizes your center of gravity. It may take some practice, but you may find a greater percentage of accuracy once you master the technique.