How to center a (background) image within a div?

try background-position:center;

EDIT: since this question is getting lots of views, its worth adding some extra info:

text-align: center will not work because the background image is not part of the document and is therefore not part of the flow.

background-position:center is shorthand for background-position: center center; (background-position: horizontal vertical);

#doit {
    background: url(url) no-repeat center;
}