This is a complicated question, because I think that the definition of success is idiosyncratic. But in my opinion, the best artists out there are the ones who completely throw themselves into what they are doing…living and breathing their creations so that they are completely engaged. I kind of think of it like method acting. I think that giving yourself away to a practice to a point of total immersion is really hard to do, but that is where the most genuine works can come from. So I guess that is really my answer to this question–a successful artist is genuine, and really feels what they are doing.
Side note: A successful artist is a happy artist. Not necessarily always in a good mood, but I think being successful can simply be an artist who is doing what they love.