My top three.
People running the processing and developing operations Ive used over the years and one in particular was especially helpful. When I got clinkers they could usually be used to figure out why, often with the help of the people that ran the lab.
Experimentation. I have never really been afraid of the fact that something might not work (well at least not till it was for a client then you don't guess) as long as I learned something from the experience. I learned a lot from building many things I use in photography. Many times the construction process taught me why things were like they are more often than not. Many would be amazed at some of the things I have used to make a photo. Anyone ever use a paper plate for a bounce card for your portable flash? The ovalish shape it provides actually makes for a nice light. That experimentation also includes subject matter. It is amazing how much the skills you develop in once area of photography, transfer and improve your work elsewhere.
Books. Whose? Well just about anyones and magazines too. Sometimes I think these people are nuts. Other times I think, "Why didn't I think of that?" but regardless of which way it works out it does one very important thing. It makes me think. As long as you are thinking, the odds are pretty good the quality will get better.
Park yourself in front of a world of choices in alternative vehicles.
Visit the Yahoo! Auto Green Center.