Be genuinely happy with your dog when you reward him. Let him know how proud you are of him, and mean what you say.
The words you praise your dog with don't have a real value, unless you really mean what you say. On the other hand, you can say almost anything, and your dog will interpret it as a praise, if the tone of your voice and your gestures show that you are happy.
Not all people are natural talents in adding emotion to their words, but fortunately praising a dog is a skill that you can practice and get better at every day!
Remember that the praise is for your dog - don't worry about what other people may think of you. Most likely your positiveness will make someone else's day, too! When it comes to rewarding, the only opinion that counts is your dog's.
Have fun training and enjoy every moment with your best friend!