Remember that when you give a job/invited talk somewhere, people don't have to come hear you. They're taking their time to listen to you, so you'd better give them your undivided attention till the very end.
And don't forget to thank the audience (however large or small) for coming, at the beginning and end of your talk. It's also a good idea to thank your host/sponsor.
The above doesn't apply to conference/workshop talks when your time is limited: in that case it'd be unethical to cut into the next speaker's time slot (or worse, a coffee break :-)
(Last updated: 11/22/2005)