In a recent discussion about e-mail subject lines I was asked by a colleague
if she should always put the client's name in it. "I want the recipient to
know it's about my client," she said. "No," I snapped a bit too tartly, "you
want the recipient to open the e-mail." After all, the message isn't going
to matter if the e-mail isn't opened in the first place.

E-mail subject lines are the subject of great interest. Google "subject
lines," in fact, and in .22 seconds 981,000 results appear (including one
from the Romance Writing Book Club message board - for them, however, even
subject lines are all about love, decolletage, long walks along the beach,
and square-jawed studmuffins on white horses). Like love and romance,
though, subject lines are worth struggling over because the potential payoff
is so big.

Here's some advice for the subject linelorn among you, in no particular
order:

*	Write it after you've composed the body of your message - Even if
you know what you're going to write about, after you've completed the text
the tone or viewpoint may be different than what you'd planned. Waiting to
write the subject line allows you to be more specific and nuanced.
*	Summarize the thrust of the message. Example: Cocktails resched to
Friday 7 p.m.
*	Focus on what's in it for the reader: Example: Here's the data on
Iowa you wanted
*	Keep it short. A study conducted last year by e-mail monitoring
company Return Path  showed that subject lines with 49 or fewer characters
had click-through 75 percent higher than for those with 50 or more.
*	Be specific - Not "Newsletter #4" but "Newsletter #4: Tips for
spring cleaning"
*	Avoid words that "sell" like "free," "buy," and "call now" - they're
like flares to spam filters.
*	Have someone else write it - You'll be surprised at how effective
this can be.
*	Avoid dates in case it gets cut off - March 26 could appear as March
2 depending on the recipient's setup.
*	Avoid: "Hi" and "FYI"
*	Don't let your subject line be your message - It's confusing to
recipients because they think something's missing (it's like when someone
says something is attached and there's nothing there, you're, like, huh?)
*	Change the subject line if the topic of the e-mail itself has
changed, though include the original subject line in brackets if you can.
Example: "Here's your mtg info [Re: We won the account!]


source: http://wordwise.typepad.com/blog/2007/03/subject_to_chan.html