I was asked whether we had guidelines for speakers for the events we run as they would be great tips for someone that wanted to improve their public speaking skills.

Not sure this is an exhaustive list but it seems to cover most of the things that the best conference speakers have in common. Would welcome other contributions.

Written from perspective of audience but many of those things make a conference organiser's life easier too and both of those things dramatically increase the likelihood that speakers will get asked back.