First of all, let's broaden the definition of success. 'Success' is going to look different for every person.
It is going to depend on what the young person is aiming for and how close he gets to his goals. We all know that by our culture's standards, 'success' means academic achievement first. My oldest is successful by these standards. It turns out that being offered a major scholarship and avoiding taking out student loans and such, was also her goal- so that she has been successful by her own standards too.
Naturally, she has many other goals-which are not necessarily academic and she is working towards being successful in attaining these goals. She works hard, she is talented and has had some luck, so I can't take too much credit for her success.
Where I can take credit though, is for helping to nurture and support her interests and goals.
I have not directed her; she directs herself. I am there to help her define her goals more clearly and to be behind her decisions. Right from the start, we the parents have nourished the child both physically and mentally. When she was little, we fed her well with love and nature and food and books.
We have enabled great swaths of time for her to do what she wants to do (in her case read like crazy, write, compose music, dream, sleep, hang out on the internet). We have included her in our lives; valued her contributions. We have encouraged her to explore her own mind and invited her to challenge and question everything.
As a result, she knows her mind. Her keen mind is her own. She knows what she wants for herself. That might be the gift of unschooling-at least it is in our case. She knows her mind because she has had very little reason to doubt herself. When you know your mind, half the battle is won.
After that, it is a matter of sticking to what it is you are after, no matter what, because you must.
Attitude is what I am talking about.
As a teen, support translates into a ready ear, an acceptance for the person she is (except maybe the crazy untidy room!! I'll never stop trying!); genuinely enjoying the person she is.
The last thing I will say is that to me, unschooling never stops. This is because unschooling means always being ready to learn all the time.