I've just seen that some of the technique/some of your questions are answered after sign up! I'll c/c here for your convenience (without the explainations and examples available on sign up)

Five tips given on the website to get good results (I have not tested myself) :

- Break your page into smaller pieces (header = 1 .psd, same for body, footer, sidebar, etc)

- Use one text layer for each logical piece of text

- Each icon or image should be a single layer or a smart object

- Want CSS buttons? Use shapes; not images.

[edit] I know there's only four. Fifth being avoid any hacks, it's probably not understandable by someone not really very familiar with Photoshop. Or I just couldn't figure out what photoshop-design-hacks are used by actual webdesigners.