Changing Mac Themes, dock, icons, etc...

I have been searching the internet for ways to customize my imac themes with different icons, themes, etc.. I have come across some with some sites and answers but none seem to do what I want.

What is the best source for this? What do you all use and how do you do it? I am running Snow Leopard. Thanks in advance.

 

Thanks Jessica but that method does not work for me. I get up to the point where I have to paste the new icon and it does not let me paste. Those directions are for Leopard. Maybe that's why?

 

I finally got it to work. I had to change the permissions of each icon before I could change the icon image.

I just downloaded leopard docks but i'm having trouble using it. I click that I want my dock to be white, click apply, and save changes, but the change does not take place.