Manon hit the nail on the head, imo.
1. Buy a few cheap trainees that have decent potential (all-star to perennial all-star is my recommendation)
2. Buy some filler material for the non-training positions
3. Spend your money on improving your arena
Your arena is the best source of income. Take a look at the arenas for other people in your league and the league above you. If your players salaries grows quicker than your arena, that is a recipe for mediocrity.