I have a beagle that loves to get high. She has a nose on her unlike any dog I have ever seen before. Anyway, she can smell me getting pot ready before she see's it. Once I get the bong, she's my best friend, all up in my business. I share about 2 bowls with her, I just cup my hands around her mouth, put my mouth on my hands, wait for her to exhale, and then I start exhaling slowly. I usually make it last for about 2-3 inhales for her, and then she goes and stones out on the couch watching tv.
P.S. Put on the Nat Geo science of dogs, and/or the Nova one about dogs, and if your dog likes TV, it'll trip out on those shows. Mine does anyway.
P.S.S. Remember dogs can hold thier breath, the trick is to be ready to exhale, and watch the ribs or stomach, you should be able to see them breathing. Mine would hold her breath for the first 10 or so times, and it was a waste (for her). After that I waited her out (damn they can hold thier breath a long time), and blow when she takes a big breath after holding it. After a couple times of that she loved it, and nudges the bong with her nose when she wants a hit.
Take your dog out to shit and piss before you do it too, just in case they get couch locked.