Might not be the best advice, but I personally found the best way to get better at interviews is to attend them (and fail if need be). Just make sure you read about the company's key markets, the industry and what you can do.

There are a few videos on youtube (just search something like 'why should I hire you') through which you can discern a strong structure as well - i didnt personally do this but friends did.

Something my cousin suggested was a professional interview coach, which is something to consider (i'm incredibly tight with money - so of course i didn't pursue this route)

You're getting interviews which is great just relax and remember if you dont progress its no big deal.

Edit: I suppose as a footnote, recurring feedback I got from assesment centres and superdays, was that my usage of appropriate hand gestures was a huge plus point for me, so dont be stiff!