Recently, Mailer Mailer released their 1st Half of 2007 Email Metrics Report.  While we need to continually test what works best in our particular situation and not rely solely on industry averages, the report did contain some interesting information that help serve as best practices. For example,

• Subject lines with 35 or few characters outperformed longer subject lines by about 25%.  While there are merits to longer copy on web pages or even in email messages themselves, the consensus continues to be that shorter subject lines improve results.  Given the way that virtually all email programs display messages, people generally use the ‘From’ address and the subject line to evaluate which emails to delete, read next, etc.  Using 35 characters as a limit forces us to communicate why the email is relevant in a very concise manner.
• While the vast majority of opens occur within the first 24 hours of a camping being launched, emails continue to be opened months after they were originally sent.  Beyond making sure that you don’t delete pages or images relating to older campaigns, you should also consider updating pages that have expired offers with updated specials or content that is no longer relevant with new information.