Everyone has read a book or an article on "how to deliver positive reinforcement." In most cases the author has provided the rules of reinforcement; a list of potential reinforcers - tangible, intangible, social, symbolic, etc. Or the KISS rule - keep it simple stupid - for delivery.
Many of the rules suggest things to say and when to say them. Using one of the many formulas for selecting what to say, or which tangible to use (cup, t-shirt, plaque), includes an element of risk - the risk that for that person, that form of verbal or non-verbal reinforcement is not effective.