Money and Guns
Gun Store - For easy money and free guns rob the gun store. Do not pull a gun as the gun store owner is quick and will kill you before you get a chance. Instead push him and box him, if you go behind the counter and push him into the corner you can use a combo almost immediately. Then you can rob the register and take as many guns and ammo as you can carry. Run out of the store before the cops arrive and you won't acquire a wanted level. Register typically has between $600-$700 dollars3
Clothing store - Run inside and push the clerk, then rob the register, usually around $120. If you steal each type of clothing it will then be saved in your wardrobe later. Run out before cops arrive to prevent wanted level. If cops do arrive before you get out, hide behind counter and kill cops, once cops are no longer in the area, steal a new outfit to drop your wanted status and walk out of the store.
Bruski's junkyard - Drive any car to Bruski's junkard, use the crusher. There is another car parked nearby, you can use crush that car also or use it to get another car, once you come back the car will be respawned. Each car/truck that you crush earn around $400. You can do this as many times as you like. You can run into police so that they chase you to the junkyard. Take out the police then crush their cars.
M1A1 Machine Gun - Earn a 4 star wanted level and the police will have M1A1 Machine guns, kill the police and take the gun.
Lazy car fix
Any home garage
If a saved car in your garage is damaged by pressing X you can repair it to full health there without having driving to a mechanics. I think the prices might be the same for both as well.
Steal a car but get the owner arrested...
Anywhere where someone is fixing a smoking car.
When finding a parked car with its owner trying to fix the engine, enter the car when police pass by (the car wont be locked and the police shouldn't come after you) and as you start the car up the owner will run over and pull you out, this will trigger the police to come over and arrest him! This can be done over and over as the owner will use unlimited money to bribe the police.
Make money without police interest.
When you drive to Bruski's scrapyard, you will notice that there is a car parked not too far away from the crusher. Take that car and do a lap of the scrapyard. When you come back, there should be another car in it's place. Crush your car and steal that other car. Do another lap of the scrapyard and there will be again another car in it's place. Again, crush your car. Repeat this until you have the desired amount of money.
Wake Up Call Achievement
In Chapter 11, you start off at your house with a surprising visit from Henry. Preventing further reprisals, Henry asks to join your family. Later on, he's ordered to kill Leo Galante. After rushing to his house and explaining the situation you attempt to escape without being caught. You start off in Leo's office. Head immediately through the open door behind you to his bedroom straight across the hall. Turn right and you'll see another door. Open it and you'll find yourself in a hallway. Look right and you'll see some sheets that you can interact with. Activating them will trigger a cutscene showing the pair escaping from the second floor. After this, you'll get the achievement,"Wake Up Call", for sneaking out without getting caught.
Easy Money $400+ each time
Go to the junkyard and there will always be a car there. Break in and put it in the crusher. Now leave the yard and soon a delivery truck will arrive. Get in it and go back to the yard and the will be another car. Keep doing it over and over for quick cash.