I've been having some trouble evading the police when they start calling the helicopters out to spot me. Two stars and below it's pretty easy to flee in a car and either avoid the cops altogether or duck into some alleyway until they lose you.
Helicopters however, always seem to spot me when there's something (a building, a container, etc) between me and the helicopter. Sneaking or going into cover doesn't seem to help.
So what's my best bet for evading the all-seeing helicopter at higher wanted levels?