Hint: Free Vehicle Repair
Did you hijack a rare vehicle (i.e., a helicoptor gunship) and want to keep it for a while? Go to any faction HQ and make sure that the last vehicle you entered and exitted is the vehicle you want to keep (and have repaired). Accept the contract. Once the merc is outside the HQ once more, locate your parked vehicle and cancel the contract. Decline to retry the contract and your vehicle will be available with full and ammo.

Hint: Chopper Ride At Kirin-Do
After the cutscene of your character landing on Kirin-Do, quickly turn around and hijack the Transport Chopper. Though it isn't shielded from radiation, it makes getting to the Type-07 prototype and General Chang a lot easier.

Hint: Free Mi-17 Chopper
If you just need a reliably consistent (always there) chopper in the north province, try the Mafia HQ. Next to the garage there should ba a man with a dollar sign for a challenge. Go accept the challenge and fail it by leaving the chopper. Once you do, you may claim the heli normally and go about your own business.

Hint: Chopper Whopper
When in a chopper and you're out of ammo and are facing another chopper, you can lower your magnet lift and hook it onto the chopper and swing it around until it's fast enough to fling it into a nearby mountain or hill. Note: This only works on choppers smaller than your chopper.