Well as PPRuNe's resident radio broadcaster, I think I'm qualified to add my humble thoughts to this thread.
My advice would be to avoid 'aviationisms' like 'push back' and 'on stand'. I've heard crews say 'top of descent' and 'flight level' before now. It means nothing in the real world. One thing drummed into me for years as a presenter is being real and being natural, and that's the key to a good PA for me.
Humour's tricky to do also, so stay away from it unless you're a natural.
Biggest laugh I had was on an EZY flight from NCL to BRS recently, where we touched down (heavily) and the PA crackled into life...'ladies and gentlemen, welcome to Bristol, where the local time is...er....er.....(fumble)....er....(punter shouts from row in front of me "ten past eight!"....er...the time is....er, eight ten.'
Much laughter from pax.