It's been said many times that the photography of Arbus shows us how no matter how much we try, we can't capture the essence of others. Nan Goldin always remarks that she made a huge effort to not show the subjects... but try to be them, even to a point of sickness. Well, no matter what happens in the images, somehow the real ugliness of reality as it is hits you in the face in those large fine grained prints. (If you read text by her, you'll be surprised on how beautiful and poetic it is in contrast.)
A medium format twin lens camera, or a medium format SLR with a waist level viewfinder. This makes your subjects look at your head while you giggle and look at their faces from a point of view slightly lower (that's why they have that sort of sheepish look).
Very fine grained slow film. Remember, you really want to show things as they are. You have to be a bit obsessed about the truth of the image. No, not that obsessed, a bit more than that. Yes. Better... feel it torturing you when you don't reach it.
Inhuman charm. This takes lots of practice. The day that you enter a pub and a complete stranger buys you a beer, you're getting close. When you manage this every other day and they open their hearts to you when you open your mouth, you're on the right track.
Patience. Yes, a day shooting a single person is needed more than once. Or do you think people relax for the camera just with your charm? No way, you have to be there till they are so tired that they forgot about you, their daily job and the world. Giving them valium is considered cheating. Also, you'll need all that patience to hear the weird stories your subjects tell you.
Distaste for composition. Did I say composition? Ha! That's something that happens to others! If a subject does not work dead centre, it wasn't worth shooting.
A taste for the strange and nocturnal. Midgets, circus freaks, old men that think they are vampires, men dressed as women, Disney memorabilia. Yes, go for it. If after seeing your prints your mother throws them into the bin you are doing it right. If some chick that thinks she's the twin soul of Tim Burton thinks they rock, well, yes, whatever...
Master use of flash. Well, it's not only that it's a handy skill to have, but if there's something that can reveal the tiniest little ugly thing in a portrait, that's a good and powerful flash.
Art galleries really appreciate your stuff after some initial uneasynes.
Your subjects after some initial outfreaking learn to love their portraits... some of them.
Shooting can be easy and spontaneous, and you can have a good time meeting people while doing it.
The art galleries that appreciate you mostly do so when you're dead.
Commisions are few and far apart. If you are an old rich toothless lady, you probably didn't want anybody reminding you that.
People you meet are pretty, pretty weird. I mean, this is not a job for any average Joe (or Jane) to take.
You take ugly and misunderstood pictures that are fairly illuminatory for an artsy minory. Being dead is not really a requisite for this achievement, but it helps. In overall I wouldn't recommend to shoot like Arbus unless you're a complete copycat that fancy galleries can recognize straight on. I mean, to build up a career on a bunch of weird pictures is not going to get you money unless you stole the whole thing from the original... and acquiring all the charm and sweettalk is hard unless you were almost born with it. But after all, who's into this for the money?