Praise is the most important tool you have to train your dog, and is also the most difficult thing for people to remember to use during a training session.
One of the most significant motivates for dogs is praise, but what is praise? Praise is not a pat on the head or a droning monotone good. Dogs are too smart for those approaches. Verbal praise should be heartfelt and belted out in high pitched, happy tones at the exact moment the dog has succeeded at the task or command. The more exuberant you can be with your properly timed verbal praise, the faster your dog will learn.
Another form of praise that can be more powerful than verbal praise is physical praise. (The pat on the head does not fit here either.) The best form of physical praise is whatever your dog likes best. For a puppy that just learned to lay down on command may want a belly rub. Since he is successful with the down, by all means rub his little belly. After he has learned to perform the command consistently, you can gradually make him work harder for his praise. Other forms of physical praise that most dogs like are stroking under the chin, stroking the ears and patting the chest.
If you only remember one thing from dog training, let it be to verbally and physically praise your dog!