I'm currently doing the near impossible challenge of getting a team from the 23rd tier of England into the Premier League. The best way to make money I've found is to simply win cups and gain promotions.
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That's three years of steady progress. I've found that playing friendlies against bigger clubs isn't as effective as previous versions of the game, and most of the time I find that I lose money.
Concentrate on getting the best possible players for your league and you should be able to breeze through the lower leagues, while making a steady amount of money.