If a sender, clients, family, & friends included, can't tell me why I should open an email in the subject, I delete it. It's their fault for not gaining my interest and differentiating their email from the 300+ junk emails I receive daily.
Subjects like: "billing," "software update," and "Scott's computer" don't cut it. Instead, I encourage my clients and friends to be more creative in their subject lines. Example:
"Eric, please review billing for November. I need your approval by Friday."
"I've installed MindManager on David's computer. No action required."
"Jack's Tablet PC arrived; Are you available to meet with him Tues?
Of course, this rule works two ways; I'm still training myself to improve my own subject line habits.