Skin as thick as a rhino's hide! Otherwise the time wasters (which are part of the territory) can make you feel resentful.
Boundaries, be really clear in your mind what your boundaries are, in areas like- which services you want to offer, client/provider relationship, turning your work phone off etc.
Warmth/friendliness- lots of smiling! (feels a bit odd at first, I mean, who actually smiles during sex)
Something that really bit me on the bum was, I didn't expect to fancy any client, sure, I knew I would find some of them attractive (even this can be awkward at first, makes me go shy) but I thought being in 'work mode' it wouldn't happen, but hormones don't understand 'work mode'.
Confidence (I hate that word, maybe 'comfortable in your own skin' would be better?) We all have those days, menstrual bloating, 'I look like shit' days, so an ability to switch it on is good.
A lot of girls develop an alter ego, I don't have the ability to do that, so I'm 'just me', but a smilier, chirpier more eager to please version of me.
Ability to work within a time frame, we have to clock watch (discretely) to make sure the client gets everything he's asked for during the session, and to bring it to a satisfying conclusion, when I'm tired, my time perception goes out the window.
Giving a fantastic blow job.
Most guys will forgive anything if your oral skills are up to par!
There's about a million more, but this is a start.