My advise is watch videos of people doing any sort of acting, mime, or even magic and analyze what makes them good or bad. Being a "showman" may come naturally, but sometimes it takes work. While you are doing your analysis think about things that you could do that would make improve your show. Don't copy other people's gestures and actions unless it fits who you are. Of course, building your effects and squeezing every last drop out of them is almost always a great way to improve you routines, but there are other ways to make your show more interesting.
This is very sound advice. Sometimes the best way for us to cultivate our own sense of showmanship is by mimicking others. For an example, the new character I'm developing is far louder and a bit more "old queen" than I've ever done in the past... I'm modeling his outward image and flamboyance from two sources; midway pitchmen and a combination of Robert Preston in the film "Victor Victoria" and Liberace. This is not "natural" to me so I'm having to practice it by drenching myself in films, audio recordings, etc. so as to program my brain. In addition to this I am acting out those parts of said sources, so I can get down the gestures, cadence, etc.
So start off by finding entertainers (not necessarily magicians) that inspire you and whom you appreciate, who seem to fit the kind of person you wish to be seen as and start mirroring them. Though you might initially do an actual impersonation of them, this will change as you get more and more comfortable with the persona.
Then There's the Hard Part... you have to get out and just do it!
I'd recommend finding scripts for pitches such as Svengali decks and learn how to sell the hell out of them on the streets, at festivals, swap meets, etc. Then again, a season on the road with a side show (a real, circus styled back lot side show) will teach you tons about showmanship and show biz.