Oren has tagged me to answer the question in the headline. I guess the simple answer would be "Code a lot", since I really feel that's the only way one can become a significantly better developer. The more complex answer consists with subjects I don't know (or don't have much experience with) that I would like to learn. It wasn't easy coming up with this list, mainly because there I so many things I would like to learn, and I had to focus myself on the subjects that I feel that I will actually get...
You know that feeling that you have too much to learn and too little time? Well, I'm currently attempting to learn the following things: Unit Testing and TDD (using NUnit and RhinoMocks mocking) Castle Stuff ActiveRecord (and through it, a little bit of NHibernate) MonoRail (MVC framework for ASP.NET) Windsor (IoC container) Boo (python-like language for .NET) C# 3.0 and LINQ SQL Server (but just a bit) Working with Resharper (especially template authoring) I am now trying to a simple web project...