On very good days, Microsoft Word and I have a love-hate relationship. On all other days we have a loathe-hate relationship.
Unfortunately for me, the rest of the civilized world is standardized on this bloated, over-complex piece of software. Whether they like it or not, professional writers have to learn how to use Word. (But "Surely," you say, "There are good alternatives?" So far I have researched about 15 Word alternatives but they all have drawbacks that rule them out as professional writing tools. Word is such a behemoth in the marketplace, that new word processing packages can't seem to muster the development resources and money to 'make it'.)
If you are writing a long document in Word, here are a few simple things you can do to defend yourself. Things that make it more likely that your long document will not crash, reformat itself, or otherwise trash your work--usually on the day you promised to send your manuscript to the publisher.
So You Want To Write A Book In Word is an eloquent article about what you can do and how to do it. It appears to be written for Word versions between 97 and 2004, but you can easily adapt it to later versions.