Code. Code code code. Code code code code code. Then code some more.
The following is a list of things you can do when they take you fancy:
- Find a project for yourself: implement it. Do this for various project types.
- Take a look at design patterns and architectural patterns. These are a bit buzz-wordy at the moment, but understanding the rationaly design patterns will help your OO knowledge come on leaps and bounds, these will help you in your projects
- Try out different bits of the framwork technologies to solve your problem: ASP.NET, Winforms to start with; WPF ; WCF; EF or NHibernate; Parallel Extsnsions (or the Parallel bits of .net 4).Trying a few Architecural patterns will help.
- Try a few different language paradigms: .net has F# built in, but have a go at scheme or lisp if you like (all functional langauages, but the last two have better resources), muck around with prolog or clips, try out some dynamic languages. Stuff you pick up in these will help you solve problems in your "core" languages, and you will find other languages are better suited to some tasks than others.
Oh and Code :-)