I've spent countless hours finessing subject lines for e-blasts and e-newsletters. The ideas for the right seven to ten word phrase range from trite to absurd as I ponder the right combination that will make someone put down their latte and read my email. A client is launching a new product and we're developing multiple emails for at least three different audience groups. I spent some time researching this matter.
While, as I suspected, there is no consensus among the "experts" in this field there are some guiding principles. Which are as follows:
-keep subject lines brief
-be specific and stay focused on your audience and their interests
-including numbers helps (ie. top 3 tips)
-A/B testing is important
Check out these links for more on the topic.
Mail Chimp - the email distribution company shares subject lines from it's clients' emails with the highest open rates. An interesting read, but isn't helpful in all situations.
Constant Contact and Clickz - both include some of the best practices.
Do it Marketing - has actual sample subject lines. A great read if you are in a creative slump!