How to Be a Good Speaker

Bjørn's two rules of being a good speaker:

1. Love the words that you speak
2. Always have something to say

An engaged speaker is more enjoyable to listen to than a bored one. If you love the words as they leave your mouth, you are more likely to engage the audience. Caveat: we all hate someone who loves to speak - too much. I am here talking about giving a presentation, where you are expected to deliver a monolog. In dialogue, be a good listener.
If you don't have something to say, don't give a talk. As a scientist, this is the same as not having done anything, in which case you are not doing your job. But I also mean this in a more general sense: live life learning, and have your lessons to share. If not, then it's a waste, in my opinion.
I'm at the 16th Evolutionary Biology Meeting in Marseille, and I trust I don't need to say that some of the presentations don't measure up to the science behind them. And that's a shame; people being bored listening to your talk when they really should be excited about the science. It's a total myth that all one needs to do is do good science, and people will be interested in your talk. Rather, unless it is the something you are supremely interested in (which is probably only a small fraction of what you hear at conferences and seminars), then people tend to lose interest, tune out, and sometimes even feel antipathy for the speaker.
There are other things a speaker can do, but those are not my rules.
I am speaking tomorrow evening on the Impact of Epistasis and Pleiotropy on Adaptation.