I bound the edges of the neckline and sleeves with a strip of black stretch fabric. There is a little bit of give in the binding here, but it's not a stretch finish. It wouldn't be suitable for anything where you need a bit of extra room to get in and out of a garment. Fine for open necklines though.
I started by cutting a strip across the stretch. I'm going for a really narrow binding, so my strip is 3cm wide.
I pinned the strip to the edge of the fabric right sides together. I didn't have any tension in the sleeve edge, but made it fractionally tighter when I did the neckline in order to hold the weight without distorting.
I then trimmed it to half size (3 mm) all the way around.
I then whipstitched on the inside, catching the seam and the folded edge.
And a couple of photos of the edges on the finished garment.
UPDATE! I still love this method, but I have now included instructions for another (faster) binding option here.