At first I was a little overwhelmed by the cops, but I figured it out, and now I can shake up to 5 stars pretty easy as long as I'm not on a mission timer. It has the added advantage of leveling stealth, as well. You see, the key is getting out of the car!
Heading offroad works out in the desert, but what if you don't want to leave LS, or drive halfway across the map from your intended destination? Very simple. Break line of sight with the cops, and then duck into a tight spot, preferably in an alley or tunnel. Get out, turn on sneak, and take cover next to your car. Watch the sight cones on the cops, and keep moving around the car to keep it between you and them when they are looking your way. If you have more than two stars, you need overhead cover, as well, so you're better off looking for a doorway or scaffold, but the sneak/cover method still works. Takes a little practice, and you have to be careful and ready to run for new cover if you get spotted.
Also, when hiding in a safehouse, it helps to actually hide, not just stand in an empty room. In Michaels house, I run into the bathroom, let the door close and then take cover against it. That trick has worked 2 out of 3 times. in Franklins house, I shook two stars taking cover outside on the lower deck level against the wall while the cops were in the house!