In the ocean near the north side of the map, you can check underwater to find a crashed airplane. When the airplane crashed, a briefcase full of money (around $12,000) also found its way to the sea bed. You can swim to find the wreckage and the briefcase. After you collect that briefcase, you can then immediately switch to another character and then switch back to the one who was located near the briefcase.
When you do, the briefcase will have reappeared. The character who is underwater will collect the case again and his supply of air will return. You can repeatedly use this trick to quickly amass a small fortune.
If your vehicle is pretty banged up and you don’t want to have to pay to repair it, simply park your vehicle, get out of it, quick save from your phone, and reload that quick save. Your vehicle will be exactly where you left it, but will be in prime condition. Useful if you have an expensive and/or customized vehicle, because it can be expensive to repair.