How to communicate web design mockup specifications to front-end developers?

Looking for best practices for communicating website design specifications to front-end developers for slicing.

Knowing that the developers will need more information than a visual representation of the design, I am looking for useful ways to communicate detailed design specifications of our graphical mockups, for example, the pixels and padding between various elements of the design, so that they can be effectively sliced into HTML and CSS. I am using Adobe Illustrator CS5 to create the graphical mockups. We have a working relationship with our front-end developers.

The question: what in your experience are good ways, or best practices that you follow, to clearly communicate web design specifications to developers for execution on the web?