You have probably attended some excellent mathematical talks, but also some from which you didn't learn much. My experience is that there is a handful of common mistakes that account for the failure of many of the talks of the second type to achieve their aims. These notes have been written in order to point these mistakes out, and to help you avoid them. Thus this page may not help you turn a good talk into an excellent one, but it could help you avoid the little mistake that can ruin your talk. Once you realise the basic pitfalls, you will be able to prepare a reasonably good talk with little effort, and from there it is sometimes a small step to turn it into an excellent one.
How to use this page: click on each of the captions below that do not make you think "yes of course". Come back and have a quick look at this list each time you are about to prepare a new talk.
Advice on paper writing by Terry Tao