This is a very important subject, and very sensitive. Allow me to share my approach to this topic.
On the one hand, these are the secrets of the trade that keep this art what it is.
On the other hand, the art of magic would die out if new magicians wouldn't learn from the older, and pass on the secrets.
So how do I determin who I let in on the secret?
My approach is as follows:
Alot of people ask how I did a trick or if I can tell them or teach them how, what I do is screen these people out using a few questions and 'tests' to see if they're worth teaching. A few examples of questions would be:
Why do you ask?
Do you just want to know how I did that, or do you want to learn to DO that?
And about the 'tests', I would simply refuse to tell them and see if/how they react, if they keep asking, if they convince me, and how they convince- if they offer me money (I'd refuse), or if they explain how they wish to bring these reactions and joy on others. If the latter, then I'd see how serious they are.
The second 'test' is basically after they show some seriousness I say that they have to nag me, and chase me, and keep asking, and sort of project how it's always on their mind. At this stage in most cases, people forget about it and I don't hear from them again on the subject, and this is not because they feel I am laying them off or whatever, I fairly explain to them as I am explaining here in this topic how I feel about teaching magic.
This is roughly how I got into magic, through a friend who got into it the exact same way.
I think this is the most balanced way to keep the secrets safe, but also keep the art alive.
Please feel free to comment and tell me if you disagree with my approach or like it. Thank you!