Keep it professional.
Do not use abbreviations, and do not assume your pax know what a slot is etc (I personally try to explain what a slot is in laymans terms.)
Try not to lie. I try to keep my pax informed with as much truth as possible (whilst also remembering there is at least one nervous flyer on board!).
Information is paramount. Being told that your aircraft is late due to the late arrival of the inbound aircraft is complete nonsense. So I am late because I am late....great. I would rather be told nothing.
If aircraft type allows, I occasionally make the odd PA from the front of the cabin (only done if there are severe delays, or tech issues once pax have boarded, as I have found that it really takes the heat off the CC)
One other gem. I was once told that if you were experiencing an emergency, never say " don't worry ladies and gents, we practice this regularly in the sim" as this gives the impression that it happens regularly. I was told to say that " we have trained for this particular event ".
I am sure that other people will have other ideas, just my tuppence.
DISCLAIMER
A bit tipsy, excuse spelling and rambling!