Some speakers swear by videotaping their practice sessions or speaking before a mirror. That's never worked for me, but I wouldn't discourage you from trying it since I may be an exception to the rule. It really works. In other words, if you are speaking for 20 minutes, you should invest 20 hours in research, development, organizing, outlining, fleshing out, and rehearsing your presentation. It is, but it's necessary if you want to delivery a dynamite presentation.
Most people have no idea how much time is required to put together a presentation or seminar.
For that reason, I only speak on subjects where I'm sure, if I invest the time to construct a program from scratch, I'll be able to deliver the same or a similar version to other groups. The most well-known speakers have only a handful of presentations. If you will invest the time to thoroughly research your subject, you'll find that you can develop several presentations from the material you've gathered. And each had or has an entirely different style of speaking, as discussed in The Speaker article. Use the style that is most natural for your personality and your message.