In my particular case, I was lucky to have some very inspiring teachers, and to be teaching assistant in large, well-organized courses. Yes, other teachers were awful, but I learned how to teach from the ones I admire (and I hope I'm doing them justice in my work). I learned a lot from colleagues, even (or perhaps particularly) from ones in other fields. Reading a lot of different textbooks has shown different approaches, and thus to be able to select different techniques when one doesn't seem to work.
I joined here in the hope to add a bit to the toolbox (and also to help out others). Up to here it has been worthwhile, I've learned a bit and got useful suggestions.
Perhaps the most important step on the road of becomming a better educator is to just recognize that having a fancy degree doesn't mean squat. Make sure you understand where the student's thinking process goes awry, and how to straighten it up. See where the learning process gets off track, and see how to keep it running (in my courses, with mostly completely unfamiliar subject matter, the standard "cram the last two days" technique is a recipe for disaster, so in the weekly exercise session with the TAs we propose three problems, one to be solved by the TA with the class, one to be solved by the class with help from the TA, and a third one to be turned in for an aggregate 5% of the final grade. This problem is just graded as "looks like it makes sense," no detailed checking. All problems and solutions are published the same week. It made wonders to the results last term, but I'll run it a few more times.).