You might also want to make sure that you're KNITting the KNIT stitches and PURLing the PURL stitches on each row. Also, remember to check the first stitch of each row to see which stitch you should start with.
For example, if you do Knit 1, Purl 1 across the first row and end with a Purl 1; your first stitch for the next row will be Knit 1. BUT if you end your first row with a Knit 1, then the first stitch of the next row will start with a Purl 1.
Same thing with Knit 2, Purl 2. Check the stitches at the beginning of each row and make sure whether you should start with a Knit 2 or a Purl 2.