Elfie shared this with us in microsoft.public.word.newusers:

Hi, I have users who are working on a document which has grown to
enormous proportions. 50.7 MB on 36 pgs. There are a number of
graphics in it. Because of its size it became extremely unstable, so
the document has been divided. I've passed on to them the posts
from other document size issues which include checking fast save,
embed fonts, embeded graphics change to links etc. They are
currently checking the documents against the information I've
forwarded to them. They would leave the document in parts however in
their words "this has interferred with the Table of Contents and
endnotes. Ideally, we would like this to be one report in one file.
Is there another way of formatting this document so that it won't
crash again, but have all the information together. I did go through
the Word manual today and it mentioned creating a "master document",
would this be the way to go?" Is there something else we should be
looking for? Thank you

I don't want to sound as a bore, but in my experience OpenOffice
(http://www.openoffice.org) is usually much more efficient at handling
large documents.
OpenOffice is compatible with MS Office, and it's Free Software (as in
Free Speech), so there is nothing that stands in your way to at least
try it out. In the worst case you loose one hour of your precious time
(I assume you'll work on a copy of your file ;-) In the best case you
have gained a very nice piece of software.

Good luck!