I have had success with MLS but as others have said it's complicated. You definitely have to scout for young players and sometimes you just have to sell a player if they want to leave or when a cheaper replacement is ready. The AI has some issues with the salary cap so leave a roster or 2 for players that are suddenly released around registration day. Also look in Europe for players who are eligible for the USA and not playing much. Those players may not want to sign straight away but you can usually get them on loan for a season.