First of all, it has to be visible. That's not easy! Imagine a road with flashing neon signs on both sides all shouting at you and competing for attention - Fried Chicken, Luxury Rooms, Low Prices. The Web is a bit like that too. In the street, or on the Web, it's not the sign that shouts loudest that wins, it's the one with the right message delivered competently. If you are vegetarian, the visual pyrotechnics of the 'Chunky Chicken' sign are just meaningless noise.
On the other hand, in this environment, a small, more tasteful sign that says 'Fresh Salads' could easily be lost.
The important factor here is something called 'The language of the marketplace'. You first have to level the playing field so that your message is appropriate for the environment in which it will appear. The best way to be seen is to be different. An animal that doesn't want to be seen adopts the same marking as its surroundings - camouflage! To make your ad visible, try to make it as visually different from its immediate surroundings as possible.