Eszter at Crooked Timber points to some public speaking tips she wrote. Some of the advice is fairly specific to the academic conference setting, but it's all excellent.
In the Crooked Timber post, she emphasizes problems with people going over their allotted time, and mentions in passing session chairs who let them. This reminds me of one of my favorite physics conference anecdotes, reproduced after the cut:
At DAMOP a few years back, a certain guy who we'll call B., just to have a convenient name, was giving a talk in a session chaired by C., a guy from NIST (not me). The talk was..., well, let's just say it wasn't as well-organized as it could've been.
B. was running way behind, and C. started signalling him that it was time to wrap it up. B. sped up a little, but didn't show any signs of stopping, so C. stood up, and moved over next to the overhead projector. C. is a pretty large guy, so he looms fairly effectively, and B. is fairly short.
B. sort of hunched down over his slides, almost cowering, looked up at C., and said, "Can I just show one more slide?"
"No," said C., and started clapping.
Then, for reasons beyond my comprehension, C. took a question from the audience. S., also from NIST, asked "What was the one more slide going to be?"
That is the sort of question I am always tempted to plant one of my friends in the audience to ask.
Someone recently told me about one session chair who had a dimmer switch on the projector. So when a speaker started running long he would just slowly dim the projector until it was switched off. At least one speaker then ran to the whiteboard in a desperate attempt to continue his talk.
At one conference I attended, one speaker's talk went a bit long. When he was done, he said "I'll take any questions you may have." The moderator replied, "No, you won't." It was all in the timing.