I just posted an article by Rick Ezell (one of my favorite writers) called 6 Exercises to Maintain Unity in Your Church Family.
He says, "Unity is not uniformity. Uniformity comes from without. Unity is not unanimity. We will never agree on everything. In fact sometimes, we must agree to disagree agreeably. Unity comes from within; it is a matter of the heart."
He also says, "Unity, like the heart, may very well be the most important muscle in the body. " So "to keep it healthy" Rick offers 6 Exercises for your churches heart to help it maintain unity.
The other cool thing is that he has four model sermons in a series called "God's Peace Corps." Links to the sermons are in the article.