1) Pray. Don't forget to reconnect with the Lord, who's the reason you're motivated to write this book anyway. If you lose your connection with him, you'll start feeling like what you are doing doesn't matter. Look to Him and the Bible for encouragement and affirmation.
2) Do something fun. Check Facebook for a while, or look at a few ebay auctions. Maybe you can find something that would be cool to write about.
3) Change your surroundings. If possible, go on a weekend trip. If not, go to Starbucks. A change of surroundings makes the world of difference in terms of reframing your perspective.
4) Have a friend look at your manuscript. There are only so many plot holes you will be able to see, because this book is your baby. A good friend, on the other hand, won't hesitate to tear it apart and tell you what works and what doesn't.
5) Stop overloading yourself with information. Getting ready for conference season, I've been trying to educate myself as possible lately, and finally struck a breaking point where I realized there was no physical way to internalize all the information the internet has available to me. Don't get so bogged down researching that you stop doing what's more important: writing.
Hope these tips have helped you. Do you have anything to add?