Buy Safe Houses. Change CJ's shirt or pants when he gets 2 or especially 3 stars. (You can buy a garment to lose stars, too.)
Use a motorcycle so you can shoot forward as well as sideways.
I usally can't do it in the early game myself. So I wait til later in the game when I can steal a Rhino.
I have also come to think it isn't worth it, as to me it seems the foes get stronger, but you will need it to make the 100% completion (which isn't necessary to finish the story).