Lots of good advice contained in the previous postings. I would re-iterate the need to find your own style and format following the basics mentioned.
You MUST bear in mind that there will always be one or two individuals who take the 'Rainboe' attitude and who will find fault in something, if not everything you say. You can't please all of the punters all of the time. Sticking to the basics is the best way to not upset too many!
Honesty is important, but as mentioned previously, it is often better to avoid being too honest as this may worry already nervous passengers because they just don't have the knowledge or ability to see how rational your comments are in the context of flight safety.
I was once advised that if your PA makes 95% of the passengers happy with use of humour etc etc, then you have failed because 5% of them are NOT happy! Good advice and another reason to keep it simple.
When you are having 'one of them days' where nothing is working and your delay is compounding it is always best to apologise for the delay and perhaps try and get some empathy from the passengers, perhaps using the phrase mentioned above. In general, people realise that sometimes, dispite your best efforts things just won't happen and as long as you let them know WHY things haven't happened, WHAT'S going on, WHAT you are doing about it and WHEN you will speak to them and give them more information, then most people will accept that you are doing your best.
DON'T blame other agencies/ departments as this is unprofessional. Mention perhaps the problem that you are having getting the bags loaded, not the fact that the loaders have cocked up. Once I have explained the problem and apologised for the delay I end my PA with a 'thankyou for your understanding and patience ladies and gentlemen, whilst we get this problem sorted out'.
For extended delays I give a timescale for when I will speak to them again. Recently in Barcelona there were CBs all around and the runway kept being shut by the airport authority as the storms passed overhead. We were told we were number 17 for start up clearance and to expect a delay of 50 minutes. My first PA explained all this and I told the pax that I would speak to them again in 20 minutes. I then started my stopwatch and to the minute, made another PA 20 minutes later, this time in the cabin on the crew handset facing the passengers. This can really help in one of these situations as they can see you 'facing up to them' and not just hiding away in the flightdeck allowing the cabin crew to take the flack. In the past I have also mentioned about the long day that the crew have had, that they are just as keen to get home as the pax and the fact that it is not the cabin crew's fault so please remain respectful towards them after a couple of upset pax thought it their right to take it out on the crew.
After another 20 minutes I spoke again to the pax, even if there was nothing new to tell them. At least it shows them that you are keeping them informed. Our total delay in the end was 1hr 25minutes and I had made a total of 4 PAs. Just to confirm what I was saying about keeping everyone happy though, I got 3 complaints (through the cabin crew questionnaires that are handed out), for being 'unprofessional for leaving the flighdeck'.....!!! You can't win!
It just shows that most pax have an extremely limited knowledge of how we work and some just assumed that we are locked away in the flightdeck these days and if we came out the aircraft is in danger from hijack.....even on the stand.....!
The last point I would make is to certainly write down what you want to say until you are fully confident in PA making. I still often write down route details (major cities that people will have heard of only please!, not VOR beacons that you are going to cross!), ETAs in local time (after checking the Jepp so that I stand a reasonable chance of getting it right!) with a tip to mention landing time if you are late as opposed to on chocks time (especially if you are late and you are landing in Schipol with a 20 minute taxy to follow!)
Hope this helps.
PP
ps Edited to add that sometimes it is better NOT to do a PA. I have often ommited my 'Welcome Aboard' PA due to a slot restriction and, late boarding, but again in the cruise I make a point of speaking to them soonest and explaing why. On a night flight I never make a cruise PA when most are trying to sleep.