don't ride along the edge, that is the worst thing to do as people will then cut by without even slowing. hold your position in your lane as you are doing. it's their responsibility to overtake safely, and by riding defensively you will of course reduce their opportunities to pass, but they should see you as the same as a tractor or any other vehicle using the whole lane and only make a pass when it's safe to do so. don't be intimidated by anyone.