First, build a diverse team with expertise in creativity, media (paid, owned and earned) and technology.
Next, demand a willingness to collaborate. This means the traditional writer/art director team has to unlock whatever concepting room they're hiding out in and let other smart people join in the fun.
Finally, start with a deep understanding of your consumers' behavior and their journey in the category.
So what makes a cross-channel idea ... big?
It's transformative to the brand-consumer relationship;
It inspires people to think or act differently;
It is uniquely simple;
It works anywhere.
The first three criteria are familiar. The fourth point takes work. Because a big idea is channel independent. The channels simply become the tools we use to communicate and connect that idea with our audience.