Do yourself a favour and go on some photoshop courses. They're usually not cheap, but the experience is priceless. Also depends on how much you want to step up your imaging output. I did an intermediate and advanced course and found the advanced to be of most benefit. Not solely because of the course content such as selection masking techniques and smart filter enhancements, but the people I did the course with. Darn, you think you know a bit about photoshop until you talk to people that work with it close to everyday. Real wake up call let me tell you!
If doing a course seems too hardcore, get yourself a copy of Adam Block's new DVD -
Powerful Processing in Photoshop. He released it at AIC2009, so I picked up a copy while I was there. Its 9 hours worth of pure gold. It is based on CS3, but the techniques also apply to CS4 (I can testify this running CS4 x64 myself).