The best thing you can do is take your lane. Bike at least 2-3 feet into it, that way cars can't pass you with out going fully into the other lane, and it gives you dodge room if you need it. Remember you have just as much right to the road as they do.

And, as terrible as this sounds, when I first started I used headphones, if you can't hear revving it's engine behind you, you can't get scared. I'd of course never suggest it, because at least in NY, you can only legally have headphones in 1 ear while on a motorway. But it helped me a lot.