Newbie - FAQ
A smile 
That is if you've done your homework and are seeing someone who is well reviewed...
From the little I know - the only thing the girls insist on - is you are dressed clean, shaved, groomed, and ready to please...
As atl_mgr said - it depends on when and where I've been. I come in business suit, tie and oxfords - or jeans and knit shirt.
So, as long as your happy, clean, and are wearing the most important piece - the smile - they will not shut the door on you... Hmm, well, I can't promise that...
From a providers point of view. The most important thing is that you and your clothe are clean, stain free and smell nice. Guys show up from in full business suits, to jeans and a t-shirt, jeans with a nice shirt or knit sweater. Or khakis and nice shirt and knit sweater. Personally? I find a man in nice jeans and a nice shirt or light sweater very sexy, it makes him look confortable and confident like he is not trying TOO hard. I find that they also usually arrive more relaxed to the appointment than the guys in suits. And us girls with our ''womans intuition'' when a client is nervous its obvious and makes us think something is wrong or dodgy....but all in all wear what makes YOU the most relaxed. Though actually my favourite outfit on a man is a military uniform lol. But I will settle for the casual look the rest of the time 
I talk to much I am going to shut up now lol.