This trick only works after you obtain a helicopter pilot and the winch ability. When on a transport or smuggling contract that relies on you not tipping the vehicle and losing the objects inside, summon a helicopter before entering the mission assigned vehicle, as close to the vehicle as possible. Then, enter the mission vehicle. The timer should start. Get back out and take off in your helicopter. Use the winch to grab the mission vehicle and gain as much altitude as possible. If you fly relatively level, you will not lose anything. However even if you bank even slightly to get around obstacles, losses will be minimal as long as you do not actually bank the copter in a hard turn. The vehicle will get tossed about, making the copter harder to fly; but if you travel directly to the drop-off point from the beginning, it will keep its cargo. This helps you avoid nearly all ground forces. All you have to worry about are the anti-air guns and other helicopters. If worst comes to worst, simply drop the vehicle after coming down to about 20 feet or less. You will have already have saved enough time to hijack another helicopter.