A successful illustration isn't just visually appealing; it makes a strong visual statement which clearly expresses the artist's intentions. Aside from colour and composition - two fundamental properties of the visual arts - the individual style of the artist is also extremely important. But when working commercially, an additional requirement comes to the fore: content. When an art director hires an illustrator, they choose him or her as much on their ability to convey a message as on individual style.
First off, consider what the illustration needs to say. Will the viewer be able to pick up on the concept at a glance? Are there secondary elements that will support the primary visual elements and help communicate the concept more clearly? Keep all of this in mind when presenting your idea to a potential client. And don't forget about those fundamental rules of colour and composition.