The way I see it, there are three different components of this:
- Enunciation: pronouncing individual words so that they are easier to hear and understand
- Presence and delivery: nonverbal cues like body language, tone of voice, and speed to capture and retain people's attention
- Content: logically structuring your thoughts and choosing words and concepts to make them easy for the audience to apprehend
Obviously, for each of these you'd benefit from practice, and also from critically listening to/watching people who are good (and bad) at each of these. Hopefully other people will have more to say about the specifics.
I'd like to call out one technique which is underutilized but very helpful: (video)taping yourself talking so you can watch yourself. If you don't do this, you'll probably never become aware of all your bad habits, let alone be able to do anything about them.