Ideally, the result is simple, memorable, and easily identified. To help you create a good device, the heralds restrict the possible colors, poses, and arrangements of charges in your device. Nor do the heralds care what artistic style you draw your device in, as long as the style was used in the heraldry of some place or time. Good devices have as few charges and colors as possible. More often, you'll have to use more than one kind of charge, or more than two colors.
Good devices repeat themselves.
If the same kind of charge appears three places, all three are identical. Good devices make it easy to identify each charge. Other charges are drawn to make them as distinctive as possible. And all charges are drawn as large as possible while still fitting in the space available. Finally, good devices have high contrast between their parts.