Potential clients want to feel like they are your friend slowly building a relationship with you. Remember, when your teacher told you to write formally in almost all instances? Well, when it comes to marketing material, you need to stop that. Potential clients like to feel comfortable with their service providers. That means leaving out big words and using contractions when you can. After all, you weren't impressing people with your proper use of "cantankerous" anyway.
You are also not impressing anyone with very long paragraphs even though your teacher told you a paragraph must have at least three to five sentences. I'm convinced teachers only said that to stop students from being lazy and actually do their work. They couldn't possibly believe it's necessary. There's evidence of shorter paragraphs in the business world everyday. I've scene paragraphs as short as one sentence or even one word to draw attention. Besides, these days people have short attention spans and are very busy. So the shorter the better!
If you want to be really daring, take a walk on the wild and start a sentence with a conjunction. *Gasp* I know what you're thinking, but sometimes it's perfectly ok to start sentences with "or", "and", or "but". When speaking, we naturally use conjunctions at the beginning of sentences to transition. It works for marketing materials to - sometimes. Just make sure you don't over do it.