[heardtheword]

First, teach something you are interested in. This will keep you going when something is challenging. Even if you don't consider yourself a "pro" in the subject, you can always get to that level.

Find somewhere you can teach and receive feedback. I teach piano to students and several times I've been caught with a question I had to look up. The best thing you can do is say "I don't know but I'll figure it out for you". Plus, this way you also have to figure out a way to explain it in a simple method to those you are teaching.

Feedback is really important because it will be how you can affirm what you know and also give you opportunities to branch out. If you aren't getting feedback on a blog, forum, or wherever you may just need to find the right place to share your knowledge.

[Fiahil]

Great answer, I will add to this that you need a lot of patience. So yeah, if you are not interested in the subject, this gets a whole lot difficult.