Language

Language is very, very important--as your choice of words can be powerful.  Here's a couple of tips:

1. Customer vs. Passenger. I've never liked the word "Customer" in reference to aircraft, especially in the safety announcements.  Example: "Customers must comply with all lighted signs, placards, etc..."  If you are in an gas station convenience store, you're a customer.  When you are in a steel tube rushing at 35,000 feet, you're a passenger.  I think that 'Passenger' is a subtle reminder to those folks sitting in the cabin that they are actually onboard a vessel.
2. You Decide. During periods of turbulence, I would avoid saying something like, "The Captain has requested the flight attendants to sit down.  When he lets us know it's safe to get up, we'll resume our service." Why?  Because it projects the subtle image that flight attendants aren't making safety decisions for themselves.   The all-knowing, all-powerful airship Captain has stepped in and determined that the food servers can't be walking through the aisle.
   
   Instead, let them know exactly who determines flight attendant safety: the flight attendants.  Make an announcement like, "Ladies and Gentlemen, the flight attendants will be taking their seats during this turbulence.  Once we have determined it's safe to be up, we'll resume our service.  Thank you for your patience."
3. Two distinct styles. Have two different styles of PA language and delivery:  one for safety announcements, and the other for the lighter (like what side of the aircraft the Grand Canyon is on) announcements.  Make your safety language clear, to the point, and not subject to interpretation.  Make your fluff stuff more informal and relaxed.
   
   An example of what not to do for safety announcements: "Folks, we're gonna need you to sit down now and fasten your seat belts.  We're really sorry about this but it really isn't safe to be up in the aisle."
   
   An example of a better safety announcement: "Ladies and gentlemen, the seatbelt sign is now on.  Please return to your seats and ensure your seatbelt is securely fastened, low and tight across your lap. Thank you."
4. Ace in the hole. If you have a really important safety announcement to make, proceed it with, "Ladies and Gentlemen, your attention please..."  They used to do this at Dodger Stadium in Los Angeles and it was the most effective attention-getter I've ever heard.  Be very careful to deliver this one without any rises in pitch or it will sound like a desperate plea.  Also, don't overuse this announcement because you will get everyone's attention by using it!
5. Humor. Humor is fine and can be a great technique, so long as it's actually funny and not plain dumb. Just keep it out of the safety messages.

Delivery

By delivery I mean the volume, pitch, clarity, and style of speaking.  Some important tricks for better delivery:

1. Hold the microphone slightly at an angle from your mouth.  This way you won't 'pop' your P's and hiss your S's.  This is an old radio station trick that will work very well on the aircraft PA.
2. Make sure that you use your 'third ear' and are constantly aware if your volume is sufficient.  Nothing is more unprofessional than too low or too high a volume.
3. Speak with authority.  Stand slightly on your toes if that helps you keep your energy going.  That's another broadcasting trick. For live safety demo announcements, go by the book, as it's always glaringly obvious (and not very professional) when the Purser is improvising the demo.

Timing

Timing is also critical to a successful flight, so I've included a few tips on this as well:

1. Remember that every flight has a deadline: the landing.  As far as passengers should be concerned, the deadline is when the seatbelt goes on in preparation for the landing.  Always be aware of the time.  The service is over and the galleys are closed.  32B can get another glass of orange juice inside the airport, thanks.
   
   Remember to give passengers (and crew) sufficient time to get the lavatory visits completed, especially on those long international all-nighter flights going into the morning.  It will make for a much easier (and safer) cabin preparation for the entire crew.
2. Timing is also important for crew sanity.  Check and actually see what your crew is doing in economy before announcing, say, the start of a movie.  Call lights going off in the middle of a service (from people with broken headsets, audio problems, etc...) can really disrupt their work.  Best to do it either before the service begins or just as the service is winding down.
3. Meal choice announcements?  Mixed feelings about this one.  If you can make the announcement cabin specific, do it during the day only, and make it short, it can be a very good tool to get people out of the aisle.  Otherwise not sure it's worth it.
4. About night flights. All-nighter flights require a special breed of announcement (and Purser for that matter): A lot of good details up front and then SHUT UP during the flight!  Except for the safety announcements of course...  Also, let passengers know that it's a special kind of flight, "Ladies and Gentlemen, this is an overnight flight which means we do things a little differently.  I'm going to tell you the details about our flight now so that you can get some rest later on.  Here's what we're going to be doing..."  Then give them specific details and times of the service, inflight entertainment, and approximate cabin preparation time.  However, night flight or not, do not wait until the very last minute to wake them up.  It will create a frantic rush to the lavatories.  This will create two problems: 1.) You will have people in the aisle no matter what, even when they shouldn't be.  Their bladder calls and they will listen.  And 2.) Everyone seeing passengers in the aisle when the seatbelt sign is on simply reinforces an ignoring of that safety reminder.

Other Announcement Tips

And finally, some other tips:

1. You've flown a million times into BLH International Airport, but Mr. 18E hasn't.  If you know the airport, try and make a good announcement about the terminal.  Like, "Folks, we've just arrived at terminal seven.  For those of you connecting on to other United flights, you'll need to go to terminal eight. There's a connecting underground train just to your left when you enter the terminal.  It takes about 10 minutes to travel to the terminal.  Of course, be sure to check the departure monitors first!"
2. Language announcements.  They always work best when coordinated beforehand with the purser.
3. Avoid announcements that go on and on and on and on and on...  You know the type: the purser who feels it is necessary to point out the location of his old high school just off the the left (as the plane is on final approach).