Just one remark, it might have been said before, but I never saw it written, so maybe it will help. People mention morale, condition, and tactic assimilation. These are key, no doubt. But despite having all this in my save, I had mixed results (which were actually rather good considering I was managing Nottingham Forrest in the Premier League, and it wasn't catastrophic). I was feeling it wasn't up to its best however.
I then hired 1st team coaches, and arranged training so that every category (shots, defence, attack, strength, etc) were at least 4 stars (before that, I had 3 (mostly), sometimes only 2 or 2.5). I also hired a great assistant manager, with a very high note in motivation - I affected him to all training categories so his motivational speeches affect everyone, all the time (maybe it doesn't do anything, but it doesn't hurt to do it anyway!).
Now I am pretty much unbeatable. Third season in the PL, I'm top with 14 wins, 1 draw and 1 defeat (both away, vs Arsenal and Southampton) in 16. Scored 63 (4 per game average) and conceded 18 (1 per game). Suso (15 goals - 12 assists in 16 games) and a brazilian regen (12 goals and 13 assists in 16 games) are unstoppable. I am through to the last 16 of the UCL (finishing top of my group which included Real Madrid and AC Milan).
This tactic is the best I've tried (had it for the past 3 seasons). Results are insane (best I got for the past 3 editions of FM), almost as good as MrHough's match engine breaker on FM12. And despite having good players (Suso, Ramires, Barkley, Walcott, Vela), my team is far from being City, Chelsea or United level. Yet, I'm the better team. Always.
My conclusion is that is you respect simple things such as tactic assimilation, physical condition, morale, using players in their best position, training, this tactic WILL get you there. If it doesn't, something is wrong with the conditions above...
Hats off to the creator.