Show that you are pleased
Because dogs are unable to communicate with us vocally they use their other senses to try to analyze our emotions and state-of-mind. Dogs can read a great deal from our eyes and facial expressions, our body posture, the way and the speed that we move, the emotional scents we give off, as well as the tone and the emotion in our voices. This is why it is very important to show our genuine pleasure and approval when the dog gets it right - dogs are not fools, they can easily tell if we are "going through the motions" and are not really concentrating on our dogs.

But don't be TOO pleased!
However, it is just as important not to over compensate and act or sound too exciting, as this sudden surge of emotion can break the dog's concentration so that it is unable to remember what it is being rewarded for. Above all - DON'T SHOUT(!) with the emotion or excitement of the occasion. Dogs associate raised voices and emotions with stressful situations in their past (after all, we have all shouted at our dogs at one time or another when they have done something wrong - it's not a very positive thing to do but we are all human!) and the dogs will think that they are being told off rather than praised.