My Tips:
Use blur tool on different strengths.
Use burn tool with different mode (midtones, highlights, shadows)
If you fail at the tips above, read this.
![](http://www.websourceinfo.com/ebooks/images/Photoshop%20CS5%20All-in-One%20For%20Dummies%20-%202010.jpg)
It taught me nothing I didn't already know. Except for silly vanishing point which I still hate unless I'm drawing with a pencil.
Edit: If you actually are interested in the book let me know.