When doing some WCF coding not too long ago, I came across the IDesign code samples by Juval Lowry, whose work I was familiar with from MSDN magazine. IDesign just recently published a paper on WCF coding guidelines which I would highly recommend. The paper is short and sweet and cuts to the chase of many best practices for WCF that can be difficult to understand from the SDK. When working with a platform as vast as WCF, this comes in very handy.
We need the corresponding document for WPF.