I use a product called Stitching Lines (
www.marinaandcolusa.com). I find them very helpful keeping the stitching lines straight. I also find that the practice of making my squares slightly larger (1/2 inch or so), sewing and then trimming down to 4 1/2" for instance works best for me (some people call it "squaring up"). I always wash my fabric and use spray sizing or starch before I ever cut it. I like to use it after I've cut my square down as well. I am making HST's for an Edyta Sitar quilt and seem to be having good luck with this method.