I would have to say attitude. Any one can develop a pros attitude to any walk of life. The fact a pro gets paid for this is because they has a great attitude to what they are doing as a rule.
If players who are not in a position to earn money from their playing take a pros attitude to their musical situation then it becomes so much easier.
7.00 means 7.00 not 7.10 or 6.30 but 7.00 so be on time.
Have your gear set up and ready to go with minimal fuss and effort.
Always be prepared for problems and handle them, try not to make your problems someone elses. Like if you break a string and do not have a spare then waste time sorting it out, or make excuses as to why you do not have a spare...your problem.
Be honest and know what it is you have to do, do not be scared to say you do not understand something. Only a fool would do or get into something they don't understand.
Be prepared to say no and mean it.
Talk about situations you are un-happy with away from the public glare, make sure your point is noted.
Always count any money in front of the person that gave it to you so there can be no miss-understandings over what you just received.
And so on and so on, like i said it is an attitude.