Grand Theft Auto: Chinatown Wars Tips

Stealing vehicles without gaining wanted level
This hint takes away the need to hotwire cars or getting the cops on you for nicking a car/bike.

To steal a car, be on foot and find a car you want. Have your weapon as fists, (the nightstick and other short-ranged weapons may work better). Walk to the car and start punching it. After a while the driver will get out. You then can get in and the cops won't get you for stealing a car!
Note: Be careful you don't kill the driver as he/she gets out, then you'll get a wanted level. Also cars may contain gang members who will try and kill you for hitting the car.

Use the hint above to get a car. Then ram the back on the bike you want. This will knock the biker off. Then get out of the car and on the bike.
(This is only useful if you NEED/WANT a bike. As you have a car to get the bike) Another way is to punch the biker off. (This also works with baggage carriers at the airport)

I don't think the car trick works on the Ammu-nation trucks though. But I haven't really tried yet.