I watched a Dachsund over the weekend who wanted to playfully naw on my fingers while we were playing. I think the owner doesn't mind, but I did. He also wanted to bark at me when I wasn't giving him my direct attention. I agree with Paige about redirection; playing with a soft toy at the same time as you are playing with the puppy will allow the fun to still occur. The puppy/dog will then transfer some of that energy to playing by himself with the toy, giving you a chance to breathe. :) I also agree with Deb about turning your back on the negative energy; waiting for calm behavior is key.
For the barking issue, twice I put him outside of the room and closed the door when he did not stop barking with my simple command. He waiting patiently outside the door with just slight whimpering. I did not keep him out long and then started fresh when he returned into the room, realizing he and I were both in training. (One of these bark sessions was for a simple phone call with my dad!)
A dog can not begin to believe that he is in charge, it will drive him to some very bad habits. Be assertive and know what behaviors you accept and will not tolerate, then communicate these simply to the puppy/dog you are watching. A dog wants nothing more than to please you as his/her caregiver.