You guys have left out the first and most importent rule of flight instructing...
"Do what I mean, not what I say."
Seriously, you don't have to know it all to be a good instructor, you just have to know where it find it all. Remember, in any teaching situation, it's always the teacher who learns the most. The same thing applies in aviation. Getting your CFI and actively instructing for a while will teach you things about flying that you will only learn through instructing.
In my case, I had an ATP and over 3,000 hours before I got my CFI. I thought that somehow I had "dodged a bullet" by being able to go directly into Part 135 (charter) flying after I got my commercial and instrument rating. I was wrong. I ended up getting my CFI certificates so that I could keep my hand in flying while I went to school full time to finish up my degree. I learned a lot and to this day the lessons are very valuable and useful. Becoming a CFI, in my opinion it is one of the most important steps you can take in your aviation career.
This leads to the question - How much time do you need to spend as an active CFI? That's a very good question. My personal opinion is that you'll probably get about all your going to get out of flight instructing after 500 to 1000 hours. Remember, there is a big difference between 1,000 hours of experience and 1 hour of experience repeated 1,000 times. Flight instruction tends to fall into the latter category.
Enjoy it, but make sure that you're giving your students what they hired you to give them. When (or if) the time comes that you aren't, then step a side.
Lead Sled