Chances are, whatever kind of word document you're working with is very similar to a document you've already done--perhaps many such documents. Use Word's templates to pre-write the boilerplate text you regularly use in documents.
Letterheads are a perfect example of templates. Walk through this example to create a sample letterhead.
Open Word and press "alt-f" followed by "N." Word will present a list of templates to choose from. Click the "Stationary" heading, then the "letterhead" link. Scroll through the many letterhead templates and double-click the "Letterhead and Envelope" item. Word will download the template to your computer and open it.
The content that comes with this template isn't just dummy text but has instructions on how to use the template. Read these to get the best use of the template.
You can use the template immediately to start writing a letter. Or, revise the letter to create your own letterhead.
If you want to create the letter rather than a template, fill out the text shown in the [brackets]. This text is held in special containers called "content controls," which you can arrange to your liking.
Create your own letterhead
To create your own letterhead based on the downloaded template, open the document in design mode: click "Developer>Design Mode." Notice the many content controls for holding the date, address and other document parts.
Reformat the control on the salutation line (i.e. "Dear Recipient..."): Click on "Recipient Name," then click "Properties," which is under the "Design Mode" icon on the toolbar. Click "New style" on the "Content Control properties" dialog box, then click "Format>Font," then choose something exotic, like "Algerian." Close out all dialog boxes and return to letter-writing mode by clicking "Design Mode." Enter some text in the "Dear Recipient" content control, and notice its font is Algerian.

click to enlarge
Save your template in a convenient place: In Windows Explorer, create a new folder on the Desktop called "Mytemplates." In Word, press "control-s" and navigate to that folder. Name the template as "myletterhead.dotm," with type "Macro-enabled template."
Now, point Word to your templates folder: In Word, choose "alt-f>Word options>Advanced" and scroll down to the "General" heading. Click the "File Locations" button, select the "User Template" line, click "Modify," and point the dialog box to the template folder you just created on the desktop.
Close all dialog boxes, and close all Word documents. In Windows Explorer, navigate to the template you saved in the subfolder under the Desktop and double-click the template. Word will open a new document based on your template.
Learn more about Word's templates here.