First and foremost write code. Write as much as you can. Tackle hard problems. If you want to be a really good programmer you need to get into the guts of what you are doing. Spend a lot of time in debuggers looking at how things work. If you want to be a good programmer who really understands what is going on you need to get down to the metal and write highly async code, learn about how processors work and why SSE is so awesome. Understand threading primitives and be able to write them as well as describe what is actually happening in the processor. I could keep going here but you get the idea.
Second find someone who knows a lot more than you and learn. This relationship will work better if you are already deeply immersed in writing lots of code.
Third, spend some time in a large high quality open source code base. I learned a ton from the Quake I and Quake II code. Helped me be a better programmer.
Fourth take on hard problems. Push your limits. Build things that you thought were impossible. Right now I am writing a specialized compiler. I have learned so much just working on this for the last couple of months.
