Practice.
You've done some interviews. You know what questions the ask. Write them on some flashcards. Pick a random card from time to time, and answer it.
This helps in 2 ways. Firstly you'll be better prepared for common questions. Secondly the process of figuring out how best to answer that question will help you figure out other questions faster.
Try not to sound rehersed. The goal is not to write a script. The goal is to listen, consider, respond. Remember you don't have to respond immediately. You can take a beat to consider your answer. (Take a sip of water, comment that the question is interesting, or that it's not been asked before etc.)
Sometimes deliberating and answer is just as interesting as the answer. Remember the question is just there to prompt you to talk about yourself.
Avoid "bullshit" answers. Like 'my biggest flaw is I work too hard.' Asking for flaws is a test of self-awareness. A better answer is '"I struggle with time management. I often used to work late because I couldn't leave the task unfinished. Now I understand that I was correcting that work the next day anyway, so I now end the day at a fixed time. Maybe a couple times a year I break that rule, because of a special time-sensitive thing, like a conference, but its a couple times a year, not every week."
It's a better answer because it tells them about you, tells them you're self-aware, and also sets a boundary. And it's not just a platitude you think they want to hear.
But practice. Get so you can answer and still be relaxed. Yes, you're nervous in thd interview, but being relaxed, and prepared, always improves your chances.
And remember, there are a lot of folk on the market right now, so don't be too hard on yourself. You're gonna get some rejections even after good interviews.
Good luck.
