I find that using the Featherweight bags instead of disposable made a big difference and also, go with the smaller bags, the 8 or 10 inch maximum and only fill about one third. I had to stop using the disposables because the plastic was very hard on my hands and fingers. Thinning your icing when you can makes a big difference. For most things except roses, you can use a fairly thin consistency, much easier on your hands. Use rubber bands to keep the icing in the bag, you will use less pressure holding the bag in place. Get yourself a foam type ball, the kind you can squeeze and it returns to its original shape? When you watch television or read, just keep squeezing the ball to improve your finger strength, lifting hand weights will help too. The problem too is we go from doing nothing to overdoing it. I get tendonitis in my finger joints so I can really relate.
Take more breaks and space out your decorating. Space out your baking, making icing too. For some of us, decorating and baking and making icing works out better over a few days. Some of us bake and freeze so we are not using our hands constantly over short periods of time.