"Premack's principle (Premack, 1959, 1963) states that more probable behaviors will reinforce less probable behaviors. The principle was derived from a study of Cebus monkeys by Professor David Premack, but has explanatory and predictive power when applied to humans. This is evidenced by the fact that therapists use the principle in behavior modification. Premack's Principle suggests that if a person wants to perform a given activity, the person will perform a less desirable activity to get at the more desirable activity. In behaviorist terms, activities become reinforcers. Students will be more motivated to perform a particular activity if they know that they will be able to partake of a more desirable activity as a consequence. If high-probability behaviors (more desirable behaviors) are made contingent upon lower-probability behaviors (less desirable behaviors), then the lower-probability behaviors are more likely to occur. More desirable behaviors are those students spend more time doing if permitted; less desirable behaviors are those students spend less time doing when free to act." (Wikipedia)
As a therapist I utilize the Premack principle with my clients all the time. But not just on my clients, but also with my ten year old daughter. It is a useful parenting tool. For example, I tell my daughter that if she reads for 30 minutes each day, then she may go on the computer for 1 hour. This works so well that she will read everyday for 30 minutes without being prompted. I tell client's parents that this is a good tool because it follows the concept of positive reinforcement.