It turns out, according to the DMA in the UK, that around two in three of us will routinely delete emails based on the subject line alone as we scan down a long list of emails. If you have ever needed a statistic to back up why you should put a lot of work, research and testing into subject lines, this will probably be it. It's fair to say that each consumer has his or her pet hates when it comes to subject lines. Personally, I can't stand those that start off with a "Re:" to make it sound like this is part of an ongoing conversation, which it isn't.
There are plenty of resources to read around about as to what makes a good subject line. There is obviously a huge place for the header to tap to deliver a well-targetted message to someone who is at a particular point in your sales funnel. "Knee pad special offer code: Act today" might well work well for someone who bought a skateboard from you yesterday, but will of course be completely untargetted to the vast majority of other people on your list.
Aside from using timing to guide the wording, the general tactics that appear to work the best are short subject lines that speak to a need the person is likely to have and either tap into their curiosity or their desire not to miss out on a time-sensitive offer. One marketing blogger, for example, lists his favourite subject lines with "Uh oh, you're prescription is expiring" as number one. Coming from an optician's, it's both personalised and useful as well as having a call to action based around timeliness, just for good measure.
Another list of the most-opened email headers advises email marketers to appeal to a person's self interest, the fear of missing out (FOMO) and humanity (Sorry, I was wrong) as great frames to fit subject lines around. Whichever you go for, it's well worth taking the time to see what has worked out for others, because with two in three of us instantly deleting emails with a subject that doesn't grab us, it's the only way to get in to the final third of those that stand a chance of engaging.