To easily gain money and experience, commit a crime while in front of your Safe House. When the police arrive, shoot as many as you can. When they are about to take you out enter the safe house. Heal and when you exit the police will be gone, but money they dropped will be there. Repeat as desired.
You can add a made man named Sean Devlin, the main character from "The Saboteur", which EA released that same year. You can probably find him in New York near Ritchie's tavern. Look inside the single story bar that has the red carpet and a man waiting outside.
This method is time-consuming but you won't get your crime rings owned for sure. In New York, do all missions until they let you to speak to people for favors. Talk to four people (since there are four henchmen to C-Rosato), then find their kill and do it. When they're all dead and the boss is the only one alive, do the rest of the missions as the soldiers are the ones that take your crime rings. Since they are dead, they can't do this with their families.
To easily defeat the Manganos, bomb one of their casinos in Florida and take over another of their casinos that you did not bomb. Take over one of the gun running warehouses and bomb the other. The bombed casino business will be reopened by now. Take it over before more guards are hired and wait until the bombed warehouse is open. Take over that warehouse and go to Cuba. Take over the remaining two Mangano businesses. Return to Florida. Go to the Mangano compound and bring one Bruiser, along with two people that have more than one specialty, and a Safecracker. Make sure they both know Medic so you do not have to worry about anyone getting hurt. Note: Do not let them go to New York. If they get one of your businesses there, they will start spreading rapidly. Also, if one of your businesses are attacked by the Manganos while you are going to take over a Mangano business, send in your men and simply wait outside of the business you are going to take over.