You might try duplicate stitch--I've used it a few times for stitches that don't quite look right, and it helps a lot. Go to Amy's video here, go to "Duplicate Stitch Join," and wait for the part of the video where she demonstrates weaving in ends using duplicate stitch. That basically is what you'd do to 'tighten' those loose stitches except you'll be using a separate piece of yarn.
Duplicate stitch is also great for decorating a piece of knitting. You can use yarn or embroidery floss.