My advice: get your economic training that much better than your competition so you can buy better players
Seriously, tho: that's my primary advice to how I've gotten where I am. Lots of people do cute "boutique" training to make elite players for themselves. I just churn volume (8 trainees per season) at a position (defensive) which minimizes the hit to my team. Also, I don't buy at solid but around formidable with a secondary so there's some value to the trainees, not just raw single skill since that doesn't sell as well.
That way, I can play a 3-5-2 in series, which is still easily the strongest formation and beat up on my competition with inelegant brute force. Hard to do at this level but it works lower down. Also, practically all of my players have specialties so I can load up on special event goals, but, again, that's something you see a lot more of in upper divisions.
But, basically, study the market to find where you can get good value (i.e. I buy my defenders mainly around formidable since they're still quite cheap) and use the economics to bludgeon your opponents. "Great, you have a pair of fancy wingers that you spent all that time and effort building. In the time that you did that, I churned $15M in cash from my trainees and poured it into the rest of my team including buying a winger that's not that much different than yours and I can sustain those wages for a lot longer, too".
That said, there are definitely more ways than that to be good. That's just mine because I feel I have a better natural ability to exploit the markets than I do to exploit the game engine.
SI