I found a nice blog about writing tips through my favourite lifehacker, The author has some nice tips about how to craft your email subject line so that it gets read.
- Write it after you’ve composed the body of your message.
- 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.
- 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!]
Some of the commenters disagree with some of the tips, and I think that I agree with some of the disagreements 
I like “subject only emails” they are more efficient, but as some of the commenters mentioned you need to indicate that in the subject. (I prefer <oem/> to (oem), it is geekier, though it has one extra character )
I also use FYI, but I usualy use it in the body and not the subject.
I prefer to use prefixes that ease filtering, for example, I use [OT] for any “Off Topic” mail I send to colleagues, I also use [Fun] sometimes for Jokes.
I think if the email has a relevant date information, then it should be in the subject, the suggestion of using 26March is excellent.
ofcourse never send an email without a subject
Please tell us your tips in the comments.
