1) Creativity. Well duh. But to really create something new and exciting, you have to be new and exciting. To get people to really notice you, you have to give them something unexpected. It's not all about just promoting your product. You need to make the video memorable. Like "Will It Blend", by BlendTech.
BlendTech's videos didn't become popular because of the blender, but what the blander was capable of blending. They figured out a creative way to showcase their product.
2) Don't make it an advertisement. Viral videos aren't advertisements that people want to share with their friends. Don't focus so much on your product, but create an interesting story. You need to create emotion. Like "Evolution", by Dove.
Dove was able to tell an interesting story, without selling their products. It built up emotion, creating a memorable story and positive feelings towards the brand. They kept the focal point of the video as the cause, and very subtly introduced their brand.