Jason Cranford Teague published an excellent slideshow called 9 Principles for Interactive Credibility. Although the slideshow is a bit on the long side (102 slides), it's definitely useful not just for designers but for anyone who has to communicate with clients and co-workers.
If you're in a hurry, you can chew on the gist of it noted in the list or have a look at the complete slideshow below. I recommend watching - it's worth the time!
9 Principles for Interactive Credibility