Excellent advice, above...I always learn from kraigwy's posts. But there will always be some muscular effort involved in holding up the dang rifle, and you may also want to do some strength training -- there are specific exercises you can do to strengthen the muscles involved, and this will help to make your bone-and-muscle platform more stable. I've found even a little of this to be very helpful. And improving your cardiovascular fitness doesn't hurt, either.
For a good introduction to this topic, see
this article from AccurateShooter.com. It also discusses dry-firing exercises.
Google something like "rifle strength training..." and you'll find a lot of ideas on this...