One thing is to use some sort of reinforcer survey to assure you capture as many possible reinforcers as possible. There are many surveys already available to you that others have done or you can create your own!
Second is to remember that anything that increases the probability the behavior will occur is a reinforcer. Therefore even relatively unusual items such as a nap, a favorite chair, a unusual toy, helping you with a chore, running out side in the yard, even a rock that the child likes the color of can be used as reinforcers. Therapist should keep their eyes and ears open for novel and unusual reinforcers especially in the case of clients that are difficult to teach things.
Thirdly, reinforcer sampling is a good practice. This technique of reinforcer sampling involves testing the reinforcer with the client by giving them a sample to see if they enjoy the item and even giving it contingent on a behavior to see if it increases the behavior.
REmember Reinforcement is extremely powerful in training the child or client but only if it is truly a reinforcer in that it increases the frequency of the desired behavior! :)