The absolute trick to it is learning what the key motivation is for your dog - for some dogs it is physical praise, for others it may be a tennis ball, for others, especially in high energy activities such as agility and flyball the behaviour itself is the best reward in the dog's eyes. Food rewards, in my opinion, are best used to teach and reinforce behaviours but are not the best way to reward behaviour the dog has already learned well. Some dogs respond better to very quiet vocal praise than they do to physical praise - it is all about knowing the individual you are working with and how to get the best out of them.