good management skills come from a lot of experience, diversification, honesty, and a host of other qualities. The G.M. must run the club as his own. That is why you must pay them well, and or offer profit sharing. You must also include them in planning, promotions, etc. there are tons of other things I could say, but I think you get the idea.
If you notice, clubs that don't pay well get one of two types of management. They either hire bad managers that run their club into the ground, with high turnover. Or they hire good managers who quite rather quickly because of low pay, again, high turnover.