I've run into this problem so many times, I doubt I can count the amount of frustration I've had trying to solve it. My site is a header, two columns and a footer layout, and for some reason I can't get the two columns to equal in length each time. An example of this problem would be if you look at my Toys section, you can see what I mean.
What I would love to learn how to do, is to extend the white #main down to the bottom of the page, connecting with the header regardless of the content included. I've looked through resources, and the only really relevant page I could find was part of the css-discuss list, but it didn't help me very much.
If you have any ideas, I encourage you to share them!
Update: Thanks to Stew for helping me out! What you can do is create a background image that is set up in CSS to repeat-y.