center aligning a fixed position div

Question

I'm trying to get a div that has position:fixed center aligned on my page.

I've always been able to do it with absolutely positioned divs using this "hack"

width:400px; left:50%; margin-left:-200px

where the value for margin-left is half the width of the div.

This doesn't seem to work for fixed position divs, instead it just places them with their left-most corner at 50% and ignores the margin-left declaration.

any ideas of how to fix this so I can center align fixed positioned elements?

And I'll throw in a bonus M&M if you can tell me a better way to center align absolutely positioned elements than the way I've outlined above.