When cutting long strips only cut as far as you have control over the ruler. Place your hand firmly on the bottom part of the ruler and cut to where the hand ends. Carefully walk your hand up the ruler and cut the next section.
If you don't know why you are off, do a few test cuts. Cut what you would normally cut for a certain size and then measure the cut piece. Is it too small, too large, or crooked. Then you can adjust your alignment accordingly. Some rulers are easier to align than others but they are also expensive. I'm a firm believer in figuring out what you got.