As much preparation as you can do, the best tip is to practice. There are some online resources you can use but what I have done is just open the camera on my laptop and record myself answering a question. If there is a time frame mentioned (e.g 1 minute preparation, 2 minutes to answer) then you should practice this. I rewatched my videos and practiced multiple times to improve. It's very important to look at the camera directly and not at the screen. Try to avoid saying "um" or "uh" and avoid rambling. If you finish your answer with 30 seconds to spare, it's okay to stop there instead of rambling on.
It's okay if you make mistakes throughout answering, but don't get stuck on this. Firms know that applicants will be nervous but it's important to be yourself, show your personality when answering questions, and also be confident! Good luck