tcb, From long experience of organising a three figure number of such functions, and listening to a four figure after dinner talks, the first rule is to take only half your allocated time - they really do not want to hear you, and really do want to have another drink. Of course whoever organises it should make damn sure that they still do have a drink in front of them, but most fail to do so. The second is, do not try to be funny. If it is after dinner, then they will fail to hear the punch line, or be determined that it is not funny whatever you say. Tell them what they want to hear, not what you want to say. If they do not have a drink before you begin, tell them to go and get one, and just listen to you from the bar. Those who go will not have listened anyway.
I think the best I ever gave, at an afternoon conference session, where I was supposed to tell a big group how successful I had been at investing several million pounds on a regular basis for a public organisation, was merely to say that because the previous speakers had encroached upon subsequent talks time and the session should have ended 5 minutes previously, I would merely wish them a good afternoon because I was sure they had better things to do than listen to me. Those who wanted to know more spoke to me afterwards.
Good luck, it is a thankless task.