Either through excellent design or irresistible offer. Preferably both.
Excellent design = Your banner should be a pleasure to look at, and attention-grabbing through colors. It should inspire professionalism to those who view it, and they will (usually) trust you and click it.
Irresistible offer = "Lose 80lb at home for free, today!" and lies like this. It's a great bonus if you actually can deliver what you promise. Make it sound like the offer of one's life.
Color, movement, bold fonts, big contrast
or with friendly, honest. blue or green with white, small part red or orange
or with vibrating red, yellow, green-orange
or with tits
or with surprising photo