Size and shape
In light of the variety of potential banner shapes and sizes, it's important to consider which designs work best in which banners. "We're looking at the dimensions of the space and figuring out the best way to tell that same story," Jetset's Young says. "So a 728x90, which is very wide and short, is a very different space to try to tell a story. It really supports more copy than images, because you just don't have a lot of height."

He points out that banners with dimensions more akin to TV screens, such as 300x250 banners, are best-suited to handle both images and copy. It's like having a mini-TV in the browser. But go too small, and there's only so much you can do. "A 160x100 is sort of limiting in another way because it's not as friendly to copy; it's really more image friendly," Young says.

Art design
With a lot of major brands, the look and feel of banners are often going to be dictated by previous marketing material. But sometimes, creative execs get a chance to really mix things up.
 
"One of the things that really sets good design apart from bad design is your ability to manage the typography," Carr says. "A lot of digital agencies don't care as much about it because they're thinking about what the words say, not the way the words say it." But he adds that marketers need to consider whether the words on a banner are shouting at consumers or speaking to them. Are they using a classic military font or some form of serif font? The difference a font can make when using the exact same words is striking, Carr notes. And although marketers may have to stay within their brands' fonts, it's likely that they can find complementary typography options that add something significant to the banners' messages.

Like fonts, a brand generally dictates the colors of a banner. However, designers can use complementary colors that take advantage of current trends in politics, fashion or seasons to enhance a message. Current fashionable colors include green because of its association with the environmental movement. And in October -- which is National Breast Cancer Awareness Month -- pink was a popular and meaningful color. But be aware that certain colors, such as bright neons, are eye-catching, but they are also distracting and unappealing. Lose them.