The above image is a little image I made a few years ago when I was sitting in an engineering class during undergrad and a friend of mine was about to throw a piece of extremely crumpled paper in his backpack. I took it from him and just put pen to paper and allowed it to flow as unconsciously as possible. And that in a sense is doodling, can't consciously guide the pen but just allow your imagination to take. over. I've found it the best way to get over a creative block
My undergrad notebooks sometimes had more doodles in the margins that words on a page and some pretty intricate. In the end I made great grades and after graduating decided against a career in engineering to move to more creative pursuits.
Now that I'm a design entrepreneur, business school student hopefully on my way into a career in design consulting, I suspect that all my notebooks will always have quite a few intricate doodles ;)