Lessons/instructor clinics combined with mileage. Epicski has also been a good "reality check" and I enjoy different people's perspectives on development.
Also, I, like Skierish, have found it helpful to ski with friends a step faster or better than me to make me step outside my comfort zone.
I also find, in the same vein as MDF and Xela, that books help either "fill in the gaps" or to revisit concepts from lessons--or to identify topics I want to investigate more in my next clinic/lesson. I do find books a good primary source of info when learning about off-snow activities like tuning, gear, etc. The problem with books is that they provide no feedback on performance, so readers are left to assume they've just done an exercise correctly (even if it wasn't even similar or created a negative change).
I haven't personally met any good skiers who haven't taken lessons (good being a relative term; my idea of good is anyone who skis at or above the level 3 instructor standard). That said, I do know a few good skiers who haven't paid for lessons!