All you have to do is stock up on ammo and stand in the general area of this picture:
No police ever come up to you unless they are pushed by another vehicle and this is very rarely. The entire parking lot area around here works in this fashion, I don't know about the other side of the building but I stood right there in that pic for an hour and had six stars for almost all of it....had a bazooka, 5000+ M4 rounds, a bunch of Magnum Pistol rounds (the one with the scope), 200+ laser sniper rounds, and a few other random things.
I also sniped a guy out of his tank and stole it and sat in the parking lot shooting every Barracks car that came past, but by the time you have six stars for a bit most everything is a tank.
Hey if you guys need a map ask me and I'll pinpoint this place, but the game calls it the Printworks.