My mom taught me to machine sew up the seam you're putting the zipper into - Lengthen your machine stitch to a basting stitch. Then pin the zipper in place and stitch. After it's in, take a seam ripper and rip out the seam where the zipper is. Easy, peasy. Works really well if you're sewing a long zipper into the back of a dress. This way you don't have to worry about the fabric slipping and one side of the zipper being wider than the other.