How can I improve my presentation and presentation skills?

What things can I do to impress the audience and make them understand my subject of presentation?
And, how can I make use of right words in English? Well, I am trying to improve my English also, speaking/talking specifically.
How can I make my voice loud?

I usually prepare before any presentation. I write all what I am going to speak and memorize. Or, at least, I memorize the points in the presentation. But, still I face the above things.
However, for spot subjects, I am no prepared and can’t do it right.

What can I do?
Any suggestions are appreciated.