Get yourself a 2 star wanted level. Then summon a cab by calling your cousin Roman, provided you have unlocked "Driving Mr. Bellic," and choose a destination a far distance from you. While riding, hold and you'll aim your gun out the window. Then cause as much mayhem as possible until you get up a 4 star wanted level. Then let go of and press and you'll be transported to your destination with no police around, so your level will disappear. This only works in one of your cousin's cabs, as in a standard taxi, the driver will run away if you earn a wanted level.
**Can be done after completing story mode**
Here is a video showing another method: http://ve3tro.com/gta/getaway.mp4