One of the nice things on twitter is people regularly pointing out to resources on how to communicate science effectively (be it through posters at conferences, powerpoint, twitter, etc). I will use this page to bookmark those links.
Slides and presentation
Giving a scientific talk? Check this excellent & extensive list of tips by John R Hutchinson: “Advice on Presenting Talks at Scientific Conferences” and this more tongue in cheek 18 tips for a really horrible presentation (via Retraction Watch).