Don’t take yourself too seriously. Use humor, but ensure that the humor supports your brand. Humor is very personal, and in order for most things to be funny, it will usually push the edges of NSFW (not safe for work) — and if you don’t know that acronym then you should definitely not be judging whether something is funny).
Don’t do creative by committee. If you want a viral video effort to work, you must free the hands of your agency from company politics.
Be aware that it might backfire. Inform those in the company that it may generate a lot of chatter, which is not always bad.
Don’t get discouraged. If one viral video fails, get up and make another one.