I'm also a programmer who doesn't like to stagnate, so perhaps I can offer a few tips:
1) What's your weakest area? Networking? Graphics? Regex? What is the one area that if someone asked you "I need a program that can do X" and that X scares you what is it. Now study as much as you can on that subject. Hack out a few prototypes and make it so that you understand it allot better. I used to hate Regex commands, now I use them whenever I can.
2) Study "different" languages. I'd recommend learning a "functional" language such as Erlang, Lisp, or perhaps certain aspects of Python. Get a book on "functional programming" and read it through, and then think how you can apply these concepts to your current work. Start using map() and filter() in python instead of for loops, etc.
3) If you're doing web programming, get yourself a massive set of data and start doing some number crunching. A while back I was playing EVE Online, so I fired up SQL Server Express and hacked out some market analysis routines in it. It was around 4 GB of data the server crunched through, but I learned allot about SQL Server in the mean time.
I recently was watching a lecture on Lisp and the Professor said: "Computer Science is not about computers and not about science. It's about knowledge, and how to manipulate that knowledge to obtain more knowledge" So true, so the more tools you have for manipulating and gaining knowledge, the better programmer you'll be.