When designing an outdoor sign or poster, let the final viewing distance be your guide. What reads well on the computer screen or in the coffee table photo book or magazine usually fails when viewed from '40 feet away at 40 miles an hour'.
Your motto should be "Keep it Simple, Clean and Direct". Don't judge your design from arm's length. Look at it on screen from across the room, tape a print on a wall and walk past it. Be harsh, look at it through the eyes of someone who is not 'in love' with what was just created and has no idea what the message is. Does it work? Keep your message to the point. Remember that a poster and a brochure serve different purposes.