Once you've defined your brand, consider your customers' online behaviors. What do they do? Where do they hang out? What are they looking for? Your creative needs to resonate with your customers-- mapping to their habits and their habitats, what they do and where they do it.
Example: When Burger King advertised on Heavy.com (a site that is all about video content), they sent King masks to two dozen of the site's regular contributors, inviting them to create their own commercials. To BK's credit, they didn't edit the videos, but instead ran them with a qualifying message that this was user-generated content.
The result: The target demographic embraces brand and runs with it. BK wins.

Consumer-produced videos aligned with Burger King's marketing programs and gave customer's ownership of the brand.
Next: Go niche