Set constant in Pages '06
You can use a Merge field.  

The first thing you need is a merge source.  This can either be your address book, or a numbers spreadsheet.  Ensure that the details you wish to include are available in the relevant source document.  Then enter Insert > Merge Field > Name/Email/Phone etc etc in the relevant places in your document.  This will create place markers for the relevant fields that you can populate later.

Once you have done this and created the fields, you can fill them in.  Click Edit > Mail Merge, check the settings and click merge, it should create a new document with the details all filled in.

I've not tried with the numbers file route, but I have found that the easiest way to produce a single document is:  Create a contact group within Address Book, call it Temporary, or anything you like.  Create the main document as above, and save it as your template, setting the group you just made above as your merge source.  Then each time you want to create a new one for a client/contact, just move them to be the only contact in the temporary group, do the mail merge, and hey presto, a new document tailored to a single contact/client.

From my limited testing just now I would expect a new document to be created for every contact in the group, but for some reason it's only doing the first contact for me, however this should still be fine for you, as you are not actually needing to do a real mail merge, just steal some of it's functionality.

EDIT: In addition to the above, I did some more testing, if you create a template document inserting the merge fields as previously described, then when you wish to populate the fields, just open address book up, and drag a contact onto any of the fields which will be highlighted as you drag.  When you let go, the contacts details will replace the place text for the field names and you will have your document.  The benefit of this method is that should a contact change their details you can just repeat the above.  The previous mail merge actively replaces the field markers with plain text than cannot be subsequently amended in this way.

Hope this helps!