Do they give any indication of which aspect of your interviewing is letting you down?
I'm assuming you've got the basics covered, like being neatly dressed, tidy hair and so on.
How's your body language? Firm, confident handshake? Do you appear compose, calm and confident, with an open posture? Google "body language for interviews", and you get loads of hits.
If they're talking about how you actually answer questions, google "STAR technique". Prepare a variety of answers which you can adapt and use for questions - they don't necessarily have to be from work - maybe you saved some money for a local group you volunteer with, by changing the way the buy tea and coffee. Any examples where you've saved money, made money or made processes more efficient will always be useful.
And breathe! If you start feeling nervous, focus on your breathing, making it slower and deeper. Don't feel obliged to answer questions instantly - it's okay to take a few moments to think. And if you don't understand a question, it's okay to ask for clarification - but it in your own words and reflect it back to them: "I just want to clarify, what you're asking me is..."