Message "must-haves"
Just as you have to clearly outline the "don'ts," you must also highlight the "must haves" in your creative brief. For example, advertisements for alcoholic beverages must have the phrase "Drink responsibly" somewhere in the video. Car advertisements must show seat belts being used properly. A list of "must haves" will ensure the consumer-generated videos you get back are in line with overall brand messaging.

Creative assets
Everything you provide to the creators can help them build the type of videos you want to see. There is an abundance of assets you can provide to them: logos, flash animation pieces, video clips of product usage, music tracks and demo reels, previous commercial work, etc. As an added bonus, if your videographers use the creative assets you provide, rather than finding their own music and footage, you will not have to spend hours and resources tracking down copyright information. Some companies, like my company XLNTads, provide talent and other releases for their creators to fill out for winning submissions.