Plan appropriately.
Practice.
Engage with your audience.
Pay attention to body language.
Think positively.
Cope with your nerves.
Watch recordings of your speeches.
Having positive thoughts about yourself and your speaking ability can really help you improve in your speeches. This article has a short but sweet section on positive thinking, along with other awesome advice.
I found this interesting that making yourself on a day that you have a speaking engagement that you make that day as routine as possible prior to your speech. This will help in relaxing yourself.