Taking Your Programming Skills to the Next Level?

An anonymous reader asks: "About 6 years ago I graduated with a degree in Computer Science. Since that time I've been working on and off as a programmer, however I feel that my programming skills haven't really progressed to the next level as I had hoped. I guess part of the problem is that my work environment hasn't been especially technical or challenging, so I really need to try and improve my skills independently. What strategies did Slashdot readers use to improve their programming skills Which books are useful in this area?"