Cutting accurately in the beginning is of major importance if your seams are accurate and the pieces still don't match up. Make sure your cuts are precise over the entire piece. If you are making several squares to piece, and your cuts are even the tiniest bit off on each one,, each bit adds up. Also, sometimes if you are using different quality of fabrics, some weaves might stretch more than others. Careful pressing (NOT pushing the iron back and forth-just "press") is a good habit to get into to prevent stretching when pressing