A few basic rules:
Don't try to be funny - unless you are a natural comedian (and not many of us are) it doesn't work - and remember most humour only works when people can see you
Smile when you do the PA - the pax DO pick up on that
Don't use technical terms
Don't say things like 'I'm afraid (we're a little late) ....'. The pax will pick up on the 'afraid' and become agitated
Remember that many of your passengers will have english only as a second/third language, so keep the delivery slow and clear and simple
Don't lie about anything, by all means don't tell the bear truth, but don't lie and then you can't be found out later.