Anyone with an inbox (or two or three) knows email marketing is big business. It reaches millions each year with offers for newsletters, exclusive deals and solutions to problems you probably didn't even know you had.

By 2013, the number of email marketing messages sent annually is expected to reach a high of 838 billion, according to a July report by Forrester's Julie M. Katz. Despite the low cost and the potential for high returns, Katz warns that by the end of 2009, consumers will have become increasingly immune to marketing pitches sent by email as a result of the high volume of such messages.

Obviously, this all points to one conclusion: You need to set your messages apart. And one of the best ways to do this is through useful and compelling subject lines. Check out these strategies for success, along with real-life examples of subject lines that spurred audiences to action.

The basics

The first two rules any professional email marketer will tell you are this: Keep it short and don't be afraid to use your brand name. Your "subject line" and your "from" line are the only two spaces -- at least for those emailers who don't use preview panes -- in which you get to distinguish yourself and your brand, so get to the point and do it quickly.

During this year's Olympic games, internet marketing company Lyris helped NBC generate its daily email updates. The company quickly settled on the subject line "NBC Olympics: Top Stories," rather than using content from each day's stories. That approach enabled the readers who opted in to the emails to easily locate and identify the NBC messages.

"E-Mail Marketing for Dummies"

John Arnold, author of "E-Mail Marketing for Dummies" and head of Constant Contact's marketing education division, says IT company Bek received good results with a similarly straightforward subject line. The line read: "Bek Tips: The Business Benefits of VOIP."

Lyris has used the same strategy for its own email marketing. Company director of product management J.D. Peterson says recipients responded better to "Lyris: Newsletter" than the other variations the company tried.

Professional emailers also emphasize the importance of time sensitivity and the need to give users a sense of urgency. Thane Stallings, a senior consultant at Epsilon, says it helps to include "something the consumer needs to do to take quick advantage of information." Coupons, offers for rebates and sale announcements all fall into this category.

Arnold reports that this strategy has proven successful for several of his clients. Scrapbooking site Create My Keepsake sent out a particularly successful email campaign with the subject line "Labor Day Sale -- 20% off coupon inside." The subject line relied on a familiar holiday theme and prompted the audience to open the email quickly.