According to Dr. Nicholas Dodman, a well-known behaviorist, if praise is used as a reward, deliver it in high singsong tones, which are most pleasing for the dog. Also, enthusiasm in your voice will be much appreciated. If petting is to be used as a reward, it should be in a way that the dog enjoys, such as stroking the dog's hair on the side of his face in the same direction that it grows, or scratching him on the chest. Note: Petting on top of the head is not appreciated by most dogs.
Dr. Dodman has some great tips in this article – please read 12 General Rules for Training Dogs