Adjust the talk to the audience:
When preparing a talk, you should first ascertain who you will be speaking to. Then you should decide what is the objective of your talk. A guiding principle is that while everyone in the audience has chosen to attend, it is your job to make it worth their while to listen.
Concerns of Young Mathematicians
(Curtis Bennett)
Make it easy for those who have drifted to rejoin you:
Include recapitulation points along your talk. Remember the audience of the big picture you have in mind.
Put things in context:
Provide historical background, motivations and examples in order to engage the audience to follow your line of though.
(John E. McCarthy)
Consider which details to include
While details are frequently at the crux of the matter in mathematics, they may also serve to confuse. Those having little to do with the ideas of the subject are best left out. I have been told that many details are best done "in private, between consenting adults".
Carry everyone along:
Address the first quarter of your talk to your high-school chemistry teacher, the second to a graduate student, the third to an educated mathematician whose interests are different from yours, and the last to the specialists.
(Halmos)
Never run overtime:
Ten Lessons I Wish I Had Been Taught
(Gian-Carlo Rota)
Have fun!
(Jordan S. Ellenberg)