The biggest threat to your email program today is not evolving deliverability determinants, the rise of social media or even your competitors’ email programs. No, the one thing that drags your email program down more than any other obstacle is the Delete button. Smaller than a quarter yet more powerful than the largest opt-in list on the planet, the Delete button shuts down your message before it even gets started, squandering all the work you put into copywriting, design, multi-platform optimization, segmenting and targeting.

Defying the Delete button begins with exceptional subject lines. Here are 5 ways to write subject lines that help your messages make a strong first impression so that they give themselves the chance to continue the conversation:

Short: One way a subject line loses its audience is when it takes too long to grok. A short subject line is not a guarantee for success, but at least you know that the whole thing will be read – and quickly. Short subject lines also stand out in the inbox because of all the white space that follows them, bordered above and below by so many words in longer subject lines. The Obama campaign has received a lot of press for its email program, characterized by short subject lines like “Dinner?”, “Hey,” “Hey again” and “So.”

Telegraphic: Some subscribers respond well to a subject line that tells them exactly what offer lies inside the message. In theory, a subject line like “Special Offer You Won’t Want to Miss!” could pull well since your audience needs to open the message to find out what the offer is. But so many marketers (and spammers) have abused that subject line tactic that most subscribers have become inured to it. Better to tell them exactly what is inside so that they are reading the message not to discover the offer, but to act on it. An example I received just yesterday is from the Washington Area Bicycling Association (WABA) which reads, “This Week Only – Save on Your WABA Membership”.

Personally Promising: Highly targeted messages convert better, so if you send them make sure you alert your subscribers in the subject line. For example, if you are offering last year’s conference attendees $100 off this year’s conference, use a subject line that indicates as much: “Annual 2011 Attendee: Here Is Your $100 Alumni Discount for Annual 2012”. A subject line that conveys the offer and the reason for it is a more sophisticated (and effective) form of personalization than simply inserting your subscriber’s first name in the subject line and calling it personalization.

Colloquial: Let’s face it – wading through dozens of messages in the inbox every day is a drag. But you can brighten up the task with a subject line that’s unexpectedly colloquial and even friendly. Outside Magazine sends its subscribers renewal notices by email with the subject line, “Dude, it’s time to renew your subscription!” It’s casual in tone, but it’s also clearly a commercial message. It arrests attention while at the same time moving recipients closer to action.

Disarming: With so much email and so many subject lines designed to trick people into opening, it is no wonder our subscribers have their guards up when triaging the inbox. A disarming subject line can break through because it demonstrates some empathy with the subscriber. You know what they are going through just trying to empty out the inbox, a sentiment that gives your brand an added opportunity to resonate. An example I received recently was from Netflix, who overcharged me for a service I did not request and was emailing to notify me they had refunded my account. The subject line: “Credit where credit is due.” Clever and disarming, not only did this subject line ensure this message was opened and read, but it also helped position Netflix as a company I’m happy to do business with. Subscribers – very much like normal people – respond well to humor and entertainment.

What all of these tactics have in common is that they do not rely on tricks or gimmicks to compel an open. Rather, they all respect your subscribers’ time, show some empathy and communicate candidly and clearly. Even when they are unsuccessful at driving opens, they at least make a positive brand impression which can help lift the open rate of your next message, whatever subject line it may have.