A bit old, but still valid, SAP's R/3 style guide for message texts often comes in handy.
Like:
- Don't use a period at the end of or within your error message
- Separate sentences inside you message by semicolons
- Formulate positive messages ("Enter a valid number" instead of "The number you entered is invalid")
- Write user-centered and avoid techno-babble
The rest of the guide is also an interesting read.