Create a template in the gallery/repository and select them as the basis when making a new document.
- Create your document and lay out the elements on the page
- Save this document under a name you can remember or sets it apart as a template.
- Go to the menu: Create new > From template...
- Select the "My Templates" tab
- Click Submit a template
- Select the document you just created as the template you want to use and share.
Next time you want to create a new document based on a template, you'll go back to the Template Gallery and select it from there.
To speed the last step for next time, note the URL that appears in your browser when you open up the template. It will look something like this:
Make a bookmark out of that. Next time you hit that address you'll load up the document. That's as default as you can get right now.
Keep in mind that while other people using Google Docs will also be able to use your template for their documents, they will not be able to edit it.