Here's something I've been using for the last two weeks. It's an exercise device for the hands, wrists, and arms. I don't know if it will help the darts, but it can't hurt.
Once it's in full swing it can theoretically weigh 40 lbs, but with my modest record of 10,743 RPM it's probably only weighing 10 lbs. Still, not bad for a piece of plastic the size of a tennis ball though. Further info available from http://www.powerballs.com/ and http://www.lovepowerball.co.uk/