How do I improve my communication for presentation, demos and meetings?

You need to find your unique style. Just as comedians have their unique style. Think Eddie murphy versus Robbin williams or Ricky Gervais versus Billy Connelly.
Imagine how wrong it would sound if we had Robbin Williams giving Eddie Murphy jokes and Ricky Gervais doing BIlly Connelly.
Its similar in presenting. I had assumed a sharp, quick, loud, comprehnsive approach was my fitting style. But playing back a recording of myself I realised I sounded very false and could not maintain the speed of flow which lead to big gaps. This gave the impression that I was forgetting what I needed to say. Which was actually the case as I was speaking too fast :)
Re-recording I opted for a more Calm, Confident, Assertive, Story telling style akin to how Obama communicates. It was a dramatic change! I suddenly sounded more interested and as a result more interesting.
I didn't try to hit every point I had prepared flawlessly. Instead I tried to engage with a story, happy to miss key facts and prepared lines.
I had found my new style.

In summary it is:
Calm (not slow) - don't rush through. Calmness shows composure and thoughtfulness.
Confident - no ehs or ums. Speak as if you are the expert on the subject.
Assertive - Speak as a matter of fact. Stand tall. Controlled gradual movements.
Story Telling - Start with Problem and Scene setting. Use Contrasts. Use Analogies. Use Examples.
!!! Rhythm- Keep sentences short and melodic with a repeating rhytm. Speak as if there is a ! at the end of each of these rhythms.