I just taught my brother his dog how to bark on command. I'm just very proud:D! And it is actually quite easy!
If you want you can check out my full story/video here:
Puppy Tales Studio | Dog Blog
I have a 1 1/2 year old mixed large breed rescue dog - and she is very quick to learn commands and tricks - except ones that (at least as far as I have learned) employ "guiding" her into the movement via a treat. For example roll over took a very very VERY long time (at that we are still working...