playing a lot... i just got Ronaldo by accident really (opened 1 star agent) - he is also lvl1 so I can develop him which is AWESOME. I got Zlatan in one of the specials and Neymar was included when I got the "bundle". I got Ozil and Sanchez as well and other mid 80's players... Mascherano is also excellent (probably 2nd most important after Zlatko to me). Not every player is awesome and it is important to have balanced team too. I got Zlatan at lvl 39 now or something crazy like that so he is rated 97, but if you have 80's defence it will not be that easy to score at all. This aint PES7/8.
They have special agents weekly and you can "easily" win at least two of them each week... they also regularly give away free money so you could buy few top agents as well. I suggest to wait for holidays offers and agents that give at least 30% black balls.
If you just started playing, there is a lot of money to be made by playing offline cups but again dont spend it on regular top agents, wait for 30% black ball ones!
First win of the day gets special bonus, so you can earn 3600-4000 GP there so thats not to miss. It helps if you have kids to play a game per day at least