Share your tips here:
There's easier ways but this is how I get a police car...
When you see one in a line of traffic, walk up to the side of another car in the line of traffic and press B, this will punch/push the car. (better with taxi's or cars with guys in it). 9 times out of 10 the guy/cabbie will get out, run away a little so he chases you and hits you, the police will then get out there car and chase them down leaving there car with the doors open for you to take!
You can also use a pedestrian as bait, if you want a police car but can't see one, hit a ped so they chase you, run fast (they will keep up) and when you see a police car let the ped hit you, then the same thing will happen as said above!
Don't get arrested, get killed instead, because getting arrested confiscates all your weapons and costs you more!