Make sure your players are consistently in good GS (8 or 9), have good defense (11 or 12 should be high enough, right?), don't have salaries that are too high, etc. When using enthusiasm, you want to use TIE as much as possible, as it builds up enthusiasm. Train a player (or two, if you can) to make sure that your team isn't regressing completely when it comes to aging. That's all I can think of right now.