The worst way to respond to this question is to indicate that you have done nothing to increase the output of your position at former jobs. The interviewer is looking to gain specific information when asking this question that will help them to understand what initiatives you have taken on your own in order to become more productive. This question can also tell you quite a bit about the company that you are interviewing with. If you are asked this question it can be a good indication that the employer expects employees to be as absolutely productive as possible. Furthermore, it also indicates that the employer expects employees to be proactive and take initiative on their own in order to get things accomplished.
When you answer this question you should make sure that you focus on positive things that you have actually done in the past that made it possible for you to be more productive. For example, you might state that you took a class in your own time in order to become more fluent in a second language so that you would be better able to respond to the needs of non-English speaking customers. This allowed you to be more productive in your position because it meant that you were able to help customers immediately instead of waiting for someone to come and act as a translator for you.
This is the type of situation that the employer is looking for when they ask this question. Your response should never be based on whether you have taken any steps on your own in order to be more productive, but instead detailing the specific steps that you have taken on your own. The precise way in which you answer this question will certainly vary based upon your own unique experiences, but do make sure you focus on providing a concrete example of a situation in which you took action on your own without needing to be directed by a supervisor and then relate how that action created a positive result. Finally, do be sure that you are completely honest throughout your answer to ensure that you do not run into any problems with providing a false answer and then having that discovered at a later time when a reference check is performed.