If you want to earn fast money, just set a player-transfer at 999999999. It will take a wile but it works.
Free players
Replace the first two digits of the price with zeros when players cost more than £23,000,000.
Lots of money
Go to the changing rooms. Click on the chair at the bottom left of the screen. Hire someone for every position.
Click on every person and give them a pay rise. Put it in all nines.
Then go to "Offer contract" and click accept. Their wage will be around -£13m so they'll pay you £13m every week and they'll be super happy.
Rapidly improving players
Do not buy any coaches, but still select players to train as you usually would if you had coaches. The players will improve the selected statistic by one point each week, and youth players will improve even faster. If players are left training on this statistic when they reach 99, they will keep improving to numbers over 100.