I don't know if you knit English or German-style (Continental), but as a German-style knitter I was very prone to loose purl stitches. I've actually adjusted the way I purl so that I catch the yarn "under" my needle instead of "over" it when I purl. I have to knit differently now too, but my tension is much more consistent.
There are a couple of different ways to purl here on Knitting Help: http://www.knittinghelp.com/videos/purl-stitch
I've taken to the "combined purl" method lately and like it a lot.