Note: Do not do this if you are actually working on, and wanting to finish the case missions, this is for people who have finished the case missions. Please read all of the following before actually doing it.

This isn't really a cheat, more like a glitch in the game I think.
On the "Medicine Run" mission, (this is the one after Brad gets shot in the leg by Carlito) you do not have to get the medicine to Brad. I think you still need to kill the shopping cart guy and get the medicine, but you dont have to bring it to Jessie for Brad. The timer for the mission will look like it is about to end but it stops and stays the same for the rest of the game.
The purpose of this is for people who have done the cases before and are tired of having to do the cases and all the other things: saving people, completing achievements, etc.
Otis will still call you throughtout the game so you can save people and what not.