I've been writing help text and error messages this week for a new development on the University's content management system. A couple of articles from Caroline Jarrett have been really helpful.
Great advice on the basics of what makes a usable error message, with examples of good and bad practice to illustrate her points.
Avoid Being Embarrassed by Your Error Messages - article by Caroline Jarrett for uxmatters.com
4 rules for displaying error messages from a user experience perspective - article by Caroline Jarrett for nomensa.com