Kim Werker has posted a great, nicely illustrated guide to double crochet for beginners. There is a tendency not to realize where the first stitch is supposed to go at the beginning of a new row with this stitch and making a mistake can lead to a piece that gets narrower and narrower. I speak from experience - I still haven't finished the shawl in question... I figured out the problem on my own eventually, but here's hoping other people can be saved the trouble of frogging most of a project.
Anyway, check it out!