Maintaining high game shape and enthousiasm. Don't play your key players for too many minutes in a week (try to stay under 70), or they'll get out of shape and perform less. You need to scout your opponents to decide wether you should start your key player (if you're not sure if you can win), or play him as a backup if you know you'll win anyway. This way you can keep their minutes at an ideal level for game shape. Same goes for enthousiasm; see if you have an easy matchup, if you do play Take it Easy to bring up enthousiasm and performance, and if you have a tough matchup play normal. You can also use Crunch time if you really need it to win a game before you return for a home game, but you need to check your schedule to see if you have tough matchup the next week. If you don't have tough matchups, you can use Crunch time because you won't need high enthousiasm in the games after, and you can build it back up by playing take it easy in your easy matchups.
To know if your matchup is tough or not, check your opponents roster and see what kind of players he has (high salary is usually a better player), and their game shape. If he has game shapes below respectable, he's not doing a good job and his players will perform worse than their salary might indicate. You can check his previous matches to see if he uses a strategy all the time, so you can adjust your strategy to it (for example, use a 3-2 zone against motion and run and gun schemes).
If you manage to do this all, you can outplay your opponents without having to invest heavily into expensive players. You can invest about all your money on your arena. I would advise to be careful with spending too much money on scouting and buying trainees, because scouting and training can be difficult to master, and if you can't properly train your players, investing a lot of money in them can be a waste. I'd try with some cheap young players and see if you can train them properly before buying high potential expensive players.