At the risk of stating the obvious, the better your headlines grab eyeballs, the more money you can make. In fact, even a small improvement can add significant money if your sales funnel is set up with a high priced product or two. For example, getting 5% more people to opt into your list or open an email or buy a $7 product could result in another one half to one percent buying your expensive product. And over the course of a year that could easily add thousands to your bottom line.
One way marketers used to test headlines was through Adwords, back in the day when they were super cheap. A marketer would try out 2 or more headlines and see which one pulled the best. Seasoned marketers still do this, but if you're on a small budget there is a less expensive way.
These days Twitter is the easiest and cheapest method for testing headlines, so long as you have a following. Simply choose what you think are your two best headlines and tweet them both and track the results.
First, the tweeting: To be fair you'd want to tweet both simultaneously, but that's probably not the best way. Instead, tweet them both in the morning and again in the afternoon, about 20 to 30 minutes apart. So for example in the morning you might tweet headline A at 9:00 and headline B at 9:30. Then do the same in the afternoon but reverse the order.
Track your results either by using bit.ly or a program like Hootesuite. When you've got your winner, test it against another headline, and so forth.
Now then... why use this method instead of split testing? Frankly, I prefer split testing when it comes to something like a squeeze page or a sales letter, but there are times when this Twitter method comes in especially handy. Are you sending out an important email or series of emails for a big promotion? Are you getting ready to launch a product and want to have a good idea in advance which headline will work best? Then using Twitter to test your subject lines and headlines may be the way to go.