There is not only 'an' art of persuasion but they are quite a few.....
1. Persuasion is not manipulation..Never force someone to do something they don't like.
2. Persuade the Persuadable....Everyone can be persuaded. They just have to be persuaded at the right time.
3.Time is everything...You should approach the person at the right time else, you won't get what you need.
4. You should like what you are doing...You should be able to be in a persuasive mood as in you should be interested in persuading someone and not persuade that person because you are supposed to..
That's all I remember for now...
Hope that helped... :)