I found a very simple, nice CSS-only method for making a sticky header in WordPress today. This is for Genesis but is easy to use with non-Genesis themes.
Adding a Sticky Header in WordPress
position: relative;
web design and development notes
There’s a free WordPress plugin that can do this. It’s called Sticky Header http://wordpress.org/plugins/sticky-header/
I think there are a few plugins that do it, but I was trying to avoid adding another plugin.