Managing Content

There's a project that I've been 'developing' for over a year now, and I'm looking to push on to the final stages, which includes getting the web site filled with content, and ready for use.

Basically, I've had a fully coded XHTML/CSS design created, but, at present, such is only one page. I'm now wondering what would be the easiest way to implement the design into a manageable system, whereby if I need to update one element of the design, I don't have to do so to all pages. I've worked with SSI previously, but I'd be interested to hear what else is available, or what you can suggest.

PHP, you can ue include tags.