Here's a useful article: What to Put in your B-to-B Newsletter
Every month, I put together the Effective Winery Web Sites e-newsletter for my small-but-growing list of subscribers (it's free), and this list is sure to come in handy. You may find it useful inspiration when writing your winery's monthly e-newsletter or weekly blog posts (you should have both).
Of course, what ever you write to your customers should be (1) interesting, (2) informative, and (3) useful. Tell a story!