In order to succeed, every email marketing campaign relies on passing one crucial test: People must open your mail.

The point may seem simple to the point of absurdity, but it can be forgotten when you are putting all your effort into the content of a message that no one ever reads. Before people can even begin to act on your message, your subject line has to be good enough to merit an initial click.

Here are five tips for managing the part of your mail that gets your customers to listen.

1. Save the sales pitch. The best subject lines are informative, not promotional. Promotional emails are inherently less likely to be opened than emails with useful, timely information. "Tax Tips for 2011 Giving" will be opened more frequently than "Give Now for Your Last-Minute Deductions" or "10% Off In-stock Gifts."
2. Keep it short and direct. Unless you are writing to a precisely targeted audience that welcomes additional information, try not to exceed 50 characters. Epsilon, a marketing consultant firm, analyzed 507 million messages and concluded that shorter is generally better, but added that it is equally important to put your most important information at the beginning of the subject line.
3. Avoid even the appearance of spam, especially because of spam filters. Your readers are bombarded with marketing from every direction and they are not willing to sift through huge volumes of mail to find the messages they want to read. Familiarize yourself with the words that trigger spam filters, as some of those triggers are not immediately obvious. Words like "help," "reminder," and "percent off" are among the seemingly-innocuous offenders.
4. Let the subject fit the type of message. Do not be afraid to use a very plain subject line. "April Newsletter" is just fine for an audience of newsletter subscribers. It needs no embellishment. The idea is to build a positive relationship with readers, not to push products at them.
5. Keep it typographically simple. Readers are justifiably suspicious of emails that insist on ALL CAPS to deliver their messages. Asterisks and other artful flourishes are equally suspect. None of your readers will find exclamation marks exciting!

In the end, there is no secret formula. The key is to offer something useful to your readers, something to make your message singularly welcome amid the relentless onslaught of junk.