If you want to do really well on your next interview look at these Performance Based Interview (PBI) questions and come up with answers for each of them. Hopefully the document will attach but if it does not attach then just google "Dept of VA PBI questions". You can download in excel or Word formats. I have tried to attach an Excel spreadsheet.
Go through the list and come up with answers and examples for each of them. If you are a student, there is a student tab. If you find you do not have examples to give, seek to find opportunities that will give the experiences needed to complete these questions. Once you have answered all of these questions, you will have, in your mind at least, an answer to some very common interview questions. This will allow you to be better prepared for those types of questions. Now go and practice over and over.
Next, when you have an interview, be very conscious of your own body's reaction before, during, and after the interview. Watch your nervousness, your breath, your actions during the interview, and any thing you think may be important. Then after the interview, review your actions. If you were too nervous, work on ways to calm yourself. Some useful methods include positive affirmations beforehand, breathing exercises, etc. If your speech was too fast or too nervous, work on ways to slow down and become a better speaker (Toastmasters can help). Doing this will ensure that each time you will get better until you start acing your interviews.
Another way you can improve YOUR performance is to actually be on an interview panel. Once you see the interview process from the other side of the table, it will help you in your interviews. If you do not currently have the luxury of being on one, ask your supervisor/manager/executive management if it would be possible to sit in on one as a quiet observer. Ask your boss to let you do this as a self-improvement processor as a willing volunteer, to see how the process works. Once you sit on the other side of the table and watch a parade of candidates come through you may see the same flaws that you have and how normal you are.
Did I mention practicing interviewing? Yes, you really need to practice a lot.