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.