I have never tried to teach my dogs to speak, so I don't have any real tips for you.

Tito barks like crazy when he is playing with other dogs. All I do is give him a squeaky or a ball to carry around when he is playing and it prevents him from barking. I guess I am lucky he is so ball crazy, once he gets it, he is not letting go for anything!

And my Granny taught her dog to speak by giving him a treat whenever he barked. Then he got to smart for his own good and used to stand next to the treat tin and howl until he got a cookie. He ended up training the rest of us.