friendlies with big clubs is the best way.. Try and arrange as many pre-season leagues you can with big clubs, you sell out your stadium and will hopefully get some TV money. Also, if you withdraw your reserves from their league you can get them to play tons of friendlies [I think], it's long but it's money.
Also, cup runs are very important, if you can get an away draw against United then you are set to earn a lot of cash, tbf, most Premier League clubs will earn you at least £250,000 from a cup game and that's before TV money.