It is important that you make the best use of any feedback that you receive. If you prepare yourself before asking for feedback, you can ensure that you get the most helpful information.
First of all, think over your performance carefully - are there any areas that you feel need more work; are not sure about how well you came across? Try and come up with a list of things to work on.
Are there any aspects that you are particularly interested in hearing about, for example, whether you seemed enthusiastic enough? You should make a note to yourself to ensure that you ask specific questions.
Don't just focus on the negative, you need to find out what you did well in order to repeat it next time. Ask if there was anything you did that particularly impressed the employer.
Remember that the more information you can gather, the easier it will be to improve your performance for next time.