Rule 1: Flag down your target customer in the headline. If you're selling to mothers, use the word "mothers" in your headline. Conversely, don't exclude anyone who may be a prospect for your product. If you're selling to both men and women, don't exclude men by calling out women. 
Rule 2: Promise your customers something in the headline! You can easily test which promises have the most appeal with your audience. Ogilvy's famous Roles Royce headline promised drivers a quieter and more luxurious ride.
Rule 3: Always try to make your headline news. Consumers are enticed by a new product, or a new way to use an old product, or to new improvements on an existing product. Don't shy away from using the words, "FREE" and "NEW" ... those words work.
Rule 4: Strengthen your headline with emotional words like 'LOVE', 'BABY', 'DARLING', 'FEAR', 'FRIEND' and 'PROUD'. And don't worry if your headlines sound cliche as a result. There's a reason these words became cliche in the first place (because they work).
Rule 5: More than 5 times the amount of people read a headline than body copy, so make sure to identify the brand which you're advertising in your headline. This is an easy one, but tons of ads fail at this important rule.
Rule 6: Include your complete selling promise in the headline, conveying to the customer what's in it for them. Both Google and Facebook give you enough space for long headlines. Don't be afraid of long headlines - they work better!
Rule 7: People will more likely click through and read more if you arouse their curiosity. Try to end your headline with a hook to motivate the customer to keep reading.
Rule 8: Your headline MUST be written in plain language. Don't use puns and don't play word games. Tricky headlines DO NOT work. Research has shown that customers will not stop to decipher obscure headlines.
Rule 9: Try to avoid negative words in your headlines. Research has shown that if you say something like "Tide detergent contains no dangerous chemicals", many people will miss the negative and be given the impression that Tide does contain dangerous chemicals.
Rule 10: Avoid blind headlines, the kind of headline that has no inherent meaning unless the reader reads the ad description or body copy. Most people don't read the body copy, so make sure every headline you write means something by itself.
Happy headlining! If you implement any of these rules and see an increase in click through rate, conversions and sales, please share your story!
