Most of the time, something subtle yet respecting the occasion. If you want to be pro looking, you must understand the occasion before you ever step into it. This not only applies to the attire but the gears such as camera, bags, actions etc as well. I was questioned by end user why I'm not using something more appropriate in my very 1st commercial assignment. This is something that I take with me throughout all future ones.
For instance, if you are shooting musical or art concert, I will wear collared dull color tops with no jeans. I limit my flashes and get the silent shutter feature turn on.
I also have vest, bags and gear catered to certain occasions. Places where a photographer should be stealthy you don't want to bring your brightly colored bags, pouches, camera straps, camou lens cover etc. I conceal my white lens during these occasions but beef up on events that require me to stand out. Eg. for motorsports, high traffic high volume events which I will be wearing brightly color safety vest and striking tops to signal to people there is 1 photographer at work.
I used to have different sets of gear catered to different events. Eg. a billingham bag, pouches, leather straps etc. which I bought specifically for formal events. I sold some of it after thinktank comes out since most of their bags are executive looking and are good enough to bring into formal event halls.
This habit is still with me and I still keep some bags and accessories for certain occasions even though I no longer shoot professionally. I have my timbuk2, crumpler for fun loving occasions like going to some kids b-day party, fun loving occasions etc. Sports event like marathons, rock climbing etc I will bring the sporty looking ones like lowepro.