What level are you at? Take note max bonus payout at around lvl 20 onwards stays at 15m. Your players salary combined would more or less be the same as well from lvl 20 onwards. Max value of your players (regardless of age) will be between 105m to 109m.
So once you reach that level, spend your finance prudently, take in and sell your academy players (ensure that your academy is fully upgraded in order to supply more academy players in one season) and you should have no problem keeping around 1.8b to 2.15b constantly.
Also look out in the bidding market for good value players which you can sell in the new season. But you have to log in often to get a good catch (i.e. free agent below 30yo). Another tip is that for academy players that costs tokens, i will only take them if at 12 tokens and below. Not fast trainees definitely, but purpose is to offload them at new season (make sure their quality is 1 or 2 away from next star).