1. Draw in the viewer from the very first clip of the ad: It's vital to get the viewers attention in the first 3 seconds of a YouTube ad. One reason being that a user can skip a standard TrueView In-Stream ad after 5 seconds. As well, the first few seconds of an ad are where the user is most likely to decide whether an ad is worth their time. Some strategies for getting the users' attention include using motion, color, and sound to make the video more interesting. Another strategy you can use to your advantage is surprising the viewer whether that be through visuals or words. This Dr. Squatch ad does a great job of this by proclaiming "Listen up, the soap you shower with is Sh*t"
2. Show your brands products/offerings: This best practice might seem obvious but I've seen many a Youtube ad where I didn't have a clear idea of what product/service a brand was selling. Make sure that your product/offerings are front and center with clear branding throughout the ad so the viewer knows what you're offering.
3. Provide a clear call to action: You got the user to view the first 5 seconds of your ad. Now what? You need to actually get the user to take an action. That usually entails the user clicking on the call to action button and going to your website to buy your products or look at your offerings. Make sure to include both a verbal CTA in your video ad, as well as utilizing the CTA button when creating the ad.
4. Keep the ads between 15-60 seconds: Yes I know the video I linked above for Dr. Squatch breaks this rule but the rule of thumb here is the sweet spot for YouTube video ad length is 15-60s.