I sew for a lovely tall but very narrow girl, I did what Nicole suggested and I started with a smaller size, in this case the size that fits around her chest. I then lengthened the skirt pieces by adding 3″ to the bottom of each skirt piece and redrawing the curves. I also cut the back bodice piece at the top of the button hole and added 1″ in here (by separating the two pieces 1″ and sticking another piece of paper behind them), this was a great move because it gives more armhole space/length while conserving the chest size. I added two button holes, but I don’t think she’s ever worn it on the smaller shoulder strap length. To get a hem facing that fitted, once I had the front skirt together I folded it in half and traced off the curve tot he depth of the hem facing and then used this as the hem facing. If you don’t want to redraft the hem facing you can finish the bottom with bias binding. It’s much easier to add length to a pattern than to remove width from the bodice.
Here’s a photo of the dress.
http://www.flickr.com/photos/73669617@N07/7305705826/in/set-72157631360905590
There’s a few others in my photo stream [just be aware there is also a matching dolls dress too :)]