When it comes to giving a presentation, split your preparation time into two stages:
Creating the material.
Rehearsal ONLY.
Changing the content right up to the last minute - which I reckon most people do - does NOT improve the presentation, because you're not investing time in actually saying the words, and that's what you are preparing for, not just reciting some wonderful wordsmithery.
If you're going to cook a meal to impress someone, do you only practise by just prepping the ingredients repeatedly for days on end? Or by prepping them and then actually cooking the thing each time, learning from your mistakes so that you can keep your cool when things do go a little astray on the big day (which they will, in some form)?
This approach should be used for talks of any preparation time. Got six months? Good, after three months, stop changing the material.
Create your lovely presentation. Then step away from it. Stop tinkering! And then REHEARSE.
It's a tough discipline, but I believe it will help you enormously.