In my line of work, I end up doing a lot of presentations. I've also studied a lot of information on how to become a better presenter. A long time ago, I took a class from Distinction Communication, a company that helps people develop more effective presentations (and presentation skills). Since then, I've taken "refresher" courses from the same folks. You can find a lot of free, helpful presentation tips and videos on the Distinction site, by the way.
Seeing what you don't see
One of the most effective techniques I learned was to videotape yourself presenting so you can see how you appear in front of an audience.
I must say, the feeling is both discomforting and extremely powerful. Not only do you get to hear how clearly (or unclearly) you communicate, you also get to see the nervous habits you don't notice from inside your own body.
For example, you may notice you use a lot of filler words, like "Um" or "Uh." You may find that you rock back & forth a lot, or don't make eye contact with your audience. You may also find that you make strange gestures that distract from your content.
All of these sorts of things are normal but very hard to notice when you're actively presenting.
DIY presentation skills improvement
For a long time, the only time I ever had my presentations recorded was during these training / coaching sessions. Last year, I realized I could do this kind of "presentation audit" myself pretty easily using tools that were readily available to me.
If you want to record your presentation, you can easily do it using your smartphone, tablet (iPad, Nexus 7, etc.), or the webcam on your computer. Lately, I've been practicing my presentations the night before in my hotel room and using my recorded session to identify problems with my presentation. I find it very helpful.
The key is to be in full "show mode" -- from a personal perspective, not just a PowerPoint perspective. In other words, pretend you have a full audience and present as though it were the real deal, as this will provide the most useful data from which to improve your skills.
It's also kind of fun to keep some of these presentation audits so you can look back and see how your skills are evolving.