The strength of the reinforcer determines whether you are able to change a behavior in your child. The following is a list of tips to assist you in discovering the most powerful reinforcers. 
1.  The first method is simply to ask your child (assuming he is verbal) what are his favorite things or activities in the world!
2.  Another technique is to talk with the child and during the conversation topics that he enjoys may come up.
3.  Observation of the child is another method. The therapist or parent can spend some time just observing the child to see what he enjoys doing!  If he plays with Legos everytime he has free time this may be a reinforcer for him. The Premack Principle in Psychology states in common words that the behavior that is frequently performed can be used to reinforcer lower frequency behavior. Therefore, observation to see what the child does most frequently during the day.
4.  Another method are to administer an inventory  or checklist which are commonly used by therapist to determine what items are reinforcing to a particular child. This can be done with the parent and or the child reporting what the want typically as reinforcers and the person records the responses on a checklist. 
5.  Reinforcer sampling first used with mental patients  inspired by Nathan Azrin is another method used to determine what items are reinforcing.  This techique utilizes a sample for the child (Ayllon, T. & Azrin, N. H. (1968). Reinforcer sampling: a technique for increasing the behavior of mental patients. Journal of Applied Behavior Analysis, 1, 13-20).

 

Parents and Caregivers may use "timers" to increase the consistency and effectiveness in reducing behavior problems such as  defiance, self-injury,  tantrums or aggression!  The timer can be an inexpensive kitchen timer or a timer on a clock, or a digital timer. the timer should be easy to set, loud enough for the child and trainer t hear and preferably portable for some behavior problems that occur in different rooms or locations. There are two major parts of the training that utilization of a timer might be helpful. The first part is during the collection of data specifying how long the behavior occurs to be counted as one incident may be crucial. For example, if a tantrum lasts for more than 2 hours it should not be counted the same as a tantrum that last for just a minute. Therefore, the caregiver might specify that each interval of  15 minutes or less is counted as one tantrum. The use of accuracy of data is important so that you know whether or not the treatment is working and whether you should change the intervention procedures. Secondly, the timer can be useful in timing the length of time to reinforce the child. For example, If the child does not know how long his reinforcement of using an ipad will last he may tantrum when you remove it in order to keep the reinforcer longer.  Therefore, it is helpful to tell the child that when the timer is finished in 20 minutes then it will be time to do another task and earn more reinforcement time or another reinforcer.  A timer is a very useful device in implementing your behavioral intervention. Even using your watch or a cheap kitchen timer can be utilized to establish whether your treatment is working, to make it clear how long the reinforcer will be delivered and assure consistency consistency consistency the application of consequences such as reinforcement. 

http://archive.weinetwork.com/shows/BrainMatters/Brain%20Matters022614-LindaMeckler-Azarin.mp3