When getting ready to slice your images for export, keep in mind that via the background properties in CSS you can control the top, bottom, left, or right placement, x and y repetition, as well as make the image non-repeating. You can also set the background image to "fixed", and it will not move with the rest of your page if it scrolls.
You'll need to look at your design and start thinking in terms of what will be exported as a complete image, and what will be used as a repeating background image. You'll probably find that your header image is the only thing that will be sliced as a whole. Many of your background images should be sliced so that their size is optimized for use as a repeated image.
If you notice that an image can repeat horizontally to get the same effect, then you'll only need to slice a small vertical area of the image. The same goes for noticing images that can repeat vertically. You'll only need to slice a small horizontal area of the image...