Easy, Dog Trick: How to Teach your Dog to Speak or Bark When you Ask.
This trick might be for more advanced dogs and takes some extra patience, as it’s one of the tougher tricks. As the video shows, you have to be diligent in waiting for your dog to bark by himself first, and then reward them as they continually do it. The dog in the video doesn’t bark immediately even though the trainer is a professional, so make sure you remember that it won’t happen right away. However, if you can master it, barking on command is a very unique trick that will definitely impress your fellow dog owners.
Source: Dogtime.com - Sarah Greenwald Video: Zak George's Dog Training rEvolution