If there is a Hospital nearby, send any injured gangsters there to heal them. This is useful if your office or hideaway is too far away. Make sure that the hospital is within your territory to be safe. If it is not, claim a nearby building that includes the hospital in the claimed turf.
It is only possible to get Aces Brannigan (the man in the black suit with white stripes holding some cards in his hand) one time at a specific place in level 15 Baylington Capture The Card Game. Go next to your office then click "Set up illegal". Hire him, and press "Relocate". If he dies, you must try again.
You can get your gangsters (especially Assassins) to pick off plenty of enemy gangsters and muscle by shooting at them, then quickly hiding in a nearby building. Most of the time, they will running, look around, then go back to their normal routine. As soon as they turn around and walk away, shoot at them again and hide. This may not always work because some of the enemies will throw your gangster out of his hiding location.