You read Notso Fantastics post, he says it all.
Here are some other thoughts:
Blend in "WE" and "US" in your pax address. This gives pax the feeling that you (cockpit/cabin) are a crew.
Think about what you would like to hear/not hear from the PA. Speak slowly, use short sentences and speak clearly.
When describing the landscape and you happen to turn your head to look outside, move the mike/handset/boomset with you.
Be truthful, open, credible, interested, motivated and effective.
Speak as if you are a winner.
Don't express opinions.
Be carefull with humour directed at any group. Use humour only if it does not affect anybody (i.e. if a situation/constellation of events is funny).
What is really nice for passengers, but also for the cabin crew, is to round off the flight after landing (while taxiing in when all CL work is done and only a few meters to roll) by bidding the pax farewell and telling them to travel safely by road (being sincere, afterall you want to see them again and the whole point of your job is transporting pax) and to then express your gratitude towards the cabin crew for there fantastic work "in front" of the pax. They love it... provided you mean it.
Don't use long sentences like these
When you speak on the PA, always put a smile on your face. This will be "heard" in the cabin... try it, it works.
Less is more
If the weather is not so fantastic (excuse the pun Notso Fantastic) and there is FOG, rather use "low clouds" or "reduced visibility" than the word FOG, as pax associate delays and other nasties with FOG.