First, take notes (either in an Engineering Journal or with OneNote) when learning a technology or skill. Pay attention to your internal questions and to the stumbling points you run across. After you have learned or acquired the skill reflect and consider what were the important concepts, what mis-conceptions did you have along the way and what could be common problems?
Next, mind map out the topics that you intend to cover in your presentation and consider relationships between the topics. Sometimes, you may opt to include that mind map with the published materials. Snapple is a nice straight forward and affordable tool for creating mind maps.
|Example Mind Map from d3.js presentation|
Then practice. Practice delivery and timing on your own. Then try to present the material internally at work, then at a local group(s) and finally at conference(s). Every time you do a presentation solicit feedback and adapt the presentation. Sometimes, the presentation has to be adapted real time because of the environment. A relaxed room of 20 people is very different then a conference hall with several hundred people in it. By the time you are presenting at a conference you will have invested a lot of time in learning and internalizing the material as well as the presentation.