One thing I do before a public speaking event is to visualise an occasion when I felt very confident and then to assume exactly the same posture/body language. The body feeds this confidence back to the brain and this usually helps to diminish any anxiety.
Also, if during your presentation you start to feel nervous again, simply pause and assume the same mindset and posture.
Works in job interviews too.
Other ways to deal with anxiety are described in this book. Unfortunately, I no longer have my copy so I can’t check but I think it covers giving a presentation.
Regardless, it’s a valuable resource and can be purchased secondhand online very cheaply.
I would also add that although I don’t think mindfulness is quite the mental health panacea it is claimed to be, this book is one of the better ones on the technique.
mindfulwaythroughanxiety.com