First think what do they want to know and what do you have to say.
The format I follow is
Pre Take-off
Good M/A/E L & G
Welcome aboard this A-PLUS Airways Boeing 737 (don't overdo- most don't know what a 737 is anyway).
Introduce the Copilot or Captain and the senior Cabin Crew member
Loading is nearly complete with all passengers on board, we'll be closing the door shortly and starting engines.
We'll be taking off from XXX airport to the south over Botany Bay.
Our route to Manderlay will take us over the Alps at our cruising altitude of 35,000 feet. Flight time will be 2 hours and 35 minutes.
The weather at Manderlay is forecast to be (be very brief).
Then the legal bit:
We are about to give you a safety demonstration about the safety features of this aircraft. It is important and contains information you must know (don't overdo the drama), so please do pay it close attention.
Seatbelts
Now please ensure your seatbelt is fastened for take-off. We advise you to fasten it even if the seatbelt sign is not illuminated in case we run into any unforecast turbulence.
Getting to the end now
Now please sit back and relax and I shall speak to you during the flight with full details of our arrival.
Cruise
Easy this one- just think 'what would I want to know if I was sitting there?'
Where we are
How high we are (for the tech heads)
How long to go
What you must do to your watch
The time in Manderlay now
The weather forecast
L & G, we are now flying at 35,000 feet over Geneva with lovely views of the Alps out of the left hand side (no more than that). We shall be landing in Manderlay in 12 hours. The local time in Burma is now 11.30 pm and you should advance/wind back your watches 7 hours to the local time of 11.30 pm.
We shall arrive on schedule at 7.45 am (get your sums right! Write down all these figures) when the weather is forcast to be snowy with a temperature of 20 degrees Celcius, 68 degrees Fahrenheit. (give them both- Celcius means nothing to Americans and lots of Brits- learn the conversion).
End- your tone of voice lets them know when you're finishing, so no weak exits- just leave it at that.
Always write down brief notes of what you are going to say and numbers- it's amazing how your brain goes empty suddenly when you need it! I find names have to be writen down in front of me or I just can't remember them. Never try and do it from memory- you will miss bits.
Never add 'thankyou' at the end- it is a very weak exit leaving people wondering why 'you' are thanking 'them'. One Captain told me 'they' should be thanking 'you' for giving them all that information!
Leave out any attempt at humour until you are an expert- you will offend somebody. Don't be too overbearing or whatever the opposite of overbearing is. Don't harp on about views- most people can't see them unless they are by a window. Don't wish they 'enjoy the flight'- how can anybody 'enjoy' being strapped into their seat hardly able to see anything?Be honest about delays- keep them informed with hopefully a glimmer of hope of getting away- don't allow deadlines to pass without saying anything. Remember some people are very nervous. Everybody has had a thought that they might not get off alive (aeroplanes do that to people), so no bad humour, no jokes about safety at all. Never use expressions like 'thunder' or 'storm'. it's just a bit 'windy' or 'rainy' or 'bumpy'- they really freak nervy people. Never lie- Sod's Law says somebody on board will know the truth! Blame ATC and there will be an Air Traffic Controller sitting down the back who will find the truth! Always point out when you are 'on time', try and leave out how late you are if you can get away with it.
Never over- apologise- one apology only otherwise people will think more and more how bad you are with people apologising all the time. Never apologise if the fault is someone else's- if you apologise, you get the blame in peoples' minds. This is Aviation Rule 1- never apologise if it's not your fault otherwise people will blame you.
Classic error just heard on GB Airways from Valencia last week- co-pilot giving interminable, mumbling PA, nobody listening anymore, me included, when I hear him mumble bored "well there's not really any more to say"........hello? Well shutup then for God's sake! Why are you wittering on? Because he couldn't think of an exit- he didn't need an exit, just shut up! Don't give those stupid platitudes like 'I hope you are enjoying your flight' (they're not). 'Thank you for flying with Snot Airways' (they'll fly with anybody if the fare is low enough). 'I wish you a pleasant flight' (you can wish to win the lottery but you won't). They all mean absolutely nothing, so don't say them.