While I completely agree with everyone who has said practice and critique are important, I would add that you need to know what the state of the field is. For example, if you're learning about drawing, practice drawing, but read everything you can about how others draw. What materials do they use? How do they generate good contrast? What is the history of the type of pencil you're using? (I'm just pulling these specific questions out of my ass, because I don't know much about drawing; probably an educated person would have better questions.) I think all these small details make a difference in understanding why people do the things they do. My craft is science, and I'll tell you I spend more time reading than doing. And I think it makes me way better at doing what I do. Whatever it is you choose to do, many others have done it before you. You should learn form their experience, as well as yours. TL;DR: Read obsessively.
Very free form. Once you find a few interesting sources in the subject you're studying, you're in business. That's because a) you develop a huge reading list from the works cited in the current work, and b) you come across things you didn't know and then you have new avenues to explore. I think you need a split between theoretical and practical knowledge, although the exact split probably depends on the field. If all you possess is practical knowledge, then you're probably missing a lot about why it is you're doing what you're doing, and you're probably being terribly inefficient, as well. If all you possess is theoretical knowledge, then you're probably sitting in a chair doing nothing but thinking about how smart you are. The good part is that if you're truly interested in the material, even dry technical crap can be very captivating. Do what you're passionate about and do it all in.
Wikipedia may get you long way these days. I'm studying to be a mechanical engineer because I'd like to be a product design engineer someday. What have I found from wikipedia? http://en.wikipedia.org/wiki/Hoekens_linkage http://en.wikipedia.org/wiki/Maraging_steel http://en.wikipedia.org/wiki/Acoustic_metamaterials I may not use any of these ever, but stuff like this keeps me motivated and helps me to know what can be done.