You aren’t powerful enough to fight the police very often. Try to avoid them. Once you’ve committed a crime, your map will mark witnesses as red triangles. The triangle’s border will fill up with red as the witness puts a call through to the police. Try to stop these witnesses before they can complete the call, preferably through non-lethal means to avoid creating new witnesses.
If you know there are too many witnesses for you to stop, leave the area. The ctOS scan will begin next, with a large circle on the map designating the overall search area, and a smaller circle showing the immediate search area. Either avoid the circles or hide.
If the police are after you, your best option is often to get out of there and avoid them until they give up. They won’t follow you into the water, so take a boat and make your escape if it becomes necessary. You may still be in danger from the police helicopter, although one unlockable skill will allow you to shut it down for a short time. If you’re in a car while being chased, try to evade and hide.
Parking lots are a good place to hide. Hack into the doors to open them, and then close them behind you. You can do the same with gated-off areas, although the gates will only slow your pursuers.
During chases near buildings, whether you are the one being chased or fled, you can hack water pipes and electric panels to slow the other person. You can blow up underground steam pipes to destroy roads. If you hack street lights, you can cause crashes and block the road.
During multiplayer, mingle with the crowd after you cause some destruction, to hide from other players by being in the middle of the crash/debris.