Here are the 4 things I did:
1. Send characters on their highest paying tasks. If you have Premium Skins for your characters use them since they pay out at a higher rate.
2. Don't waste money on items that don't pay out (unless they are needed to perform tasks).
3. If you spend Donuts only use them for more characters, buildings that pay out or items that increase your payout. There are plenty of inexpensive items that will increase your Tree Hugging, Vanity, Obedience etc...
4. Don't buy Land until you need it. Why waste money on land if you have nothing to put on it?