The single most common mistake in visual advertising is clutter, and when it comes to advertising, less is more. If there are aspects of your design that aren't absolutely essential to delivering the message, get rid of them. Chaotic, overworked ads will never be read.
This ad for children's martial arts classes is a visual assault that tries to incorporate a patterned background, photographs of children, numerous type fonts, building block illustrations, a headline, bullet list, call to action, company name, phone number and url.
Now compare that to this simple, benefit-driven message done by Banda, Bulgaria. It features only a simple background and illustration, a three-word headline and the company logo.