To get more money, take over as many businesses, warehouses, and rackets as possible before completing missions. By doing this you will get much more on payday, and money for important weapon upgrades.
To stop your money from going into some racket that will not pay off later, get the pressure up on the person you would normally buy out and he will instead pay you.
In missions where you raid an enemy gang's operation (like a supplier or storefront racket), search the interior after completing the main objective for hidden stashes of money and ammunition. Most back rooms, closets, and rooftops contain hidden moneybags or weapon stashes.
To get easy money and respect, go to the Molotov Cocktail seller in Midtown. Look for hobos walking around, then shoot them. You will receive no heat, but either +750 Respect and $3000 or +900 Respect and $100.
Different families have different incomes; the richest family is the Barzini (green) family followed by the Cuneo (red) family. The businesses that you take from these families pay out a lot more than the other two. Also, when you are taking over their business, do not forget to pick up the money they drop. When a Barzini (green) family member is killed, you can pick up at least $200 per person. However, when you take money from a Tattaglia (brown) family member, you will only get $15.
Enter the sewer system and find a crate containing money, which is guarded by a group of heavily armed bell-boys. The entrance to the sewer system is directly south of the westernmost Tattaglia warehouse on the Hell's Kitchen Loop.