Okay, stick with me on this one, I don't mean you have to abuse your reader and call them obscenities throughout your copy. If you cut through the crap, the majority of people care about themselves first and foremost. Seeing as most of the copy you'll be writing isn't aimed at the Dalai Lama, you can make the assumption that writing what I call 'Selfish Copy' is a sensible approach.

People want to know what's in it for them. If you're writing to persuade or sell then emphasizing benefits to the reader is paramount. You can write reams of copy about how great your brand is and how experienced your team are, but the truth is that people don't really care about that as much as they care about how a product/service will transform their life and turn them into the super cool person they know they are deep down. Taglines and leading sentences need to grab someone and tell them what WILL change with your product. 

Writing copy that focuses on the reader satisfies their own self importance and nothing you're going to write will mean more to someone than something that concentrates on the benefits they can enjoy.