I sat through a session by Heather Solomon at TechEd 2008 where she talked about creating custom templates for SharePoint 2007, something that definitely isn't a quick weekend project. She has some great resources to get started on that process.
- CSS Reference Chart for SharePoint 2007 - This is an amazingly detailed drill down into all the CSS options that are available for use in SharePoint. What's even better is she includes screen shots for where each CSS style gets applied.
- Minimal or Base Master Pages - As she described in her session, many of the content panes are a required part of any SharePoint template even if you're not going to use them. Here she provides the bare minimum required for any template. One important thing is that you can place unused content panes into a panel that has its visibility set to hidden.
Both of these also applies in various forms to WSS 3.0.