that what the mum says, goes ! there is a lot to be said for 'quiet support', as a fellow doula said to me. knowing what is in her birth plan / preferences, being prepared to fight her corner if necessary, and not projecting your own feelings and fears onto her.
female birth partners, who have had their own children can offer a different facet of support. i believed my mum and my best friend when they told me i could do it !
if you are told to f**k off, don;t take it personally, labour can transport a woman to a whole other place
also, once labour is established, not lots of talking, try whispering, often midwives will whisper too , dimming lights, and making it a safe and private environment.
probably loads more, but that is what springs to mind right now