I'm also managing Manchester United.
What I do, is i always ask for more affiliated clubs every season. This means that i have a greater choice of clubs to send players to and a kinda progression thing going on (half star players go to one club, next season theey go to a lisghtly better one etc..)
Hope this helps, if you want any other info, quote me, and let me know how you are doing with your game lol!
I'm just beginning forth season. Have won every trophy more than once except the Champions League, in which i seem to be incredibly unlucky every year! For example, this year, brought on young defender on for last ten mins as smalling got injured. He gave away a penalty and went out on goal difference lol!