A killer headline. The headline is the part of the ad that grabs your prospect's attention. It must be enticing and captivating. The headline is probably the most important part of your ad. Use CAPS to stress words of emphasis.
Some folks capitalize the word of focus and others capitalize their entire headline.
Just DON'T capitalize your whole ad. Caps give the impression of yelling, certainly the whole ad should not yell. Show benefits, not features. A benefit is what the product or service DOES for you while features only show you what the program or product offers. The fact you can sit on it for days without getting sore is the benefit.