Yeap~ Just be natural, but make sure it suits the genre and stuff too, like fgl says..
Like.. you don't go thrashing on stage during a ballad song, for example..
You don't necessarily need a lot of space movement to make good showmanship either, just improvise according to the songs but it helps a LOT to keep the contact with your audience, esp the ones nearest to you at your feet or something. I tend to communicate the softer parts of the songs with occasional word leads to someone in the audience, as if I'm serenading to them-- helps get their attention into the part of the song and emphasize the emotion at the same time. I'm not how the idea works but usually when I focus on one person, i tend to get the attention of the people around him as well. I don't know how it feels for them, but I know it definitely gets their attention, which is vital for me in being a front-person.
For heavier parts, again I pick up on eye contact, only with a slightly serious facial expression (I get remarks / complaints that I look fierce.. lol whatever lah I'm in a metal band liao) and maybe some hand movements towards them or a foot over the monitor if i'm not wearing a skirt (I get worried it'll tip over the people tho so I rarely do this). I don't know about other audiences but the ones I end up communicating with this way figured it's the sign that there's a headbangable moment coming lol. I love my fans. **hugs fans**
okay off topic lol. ahem k point is, you don't have to rely on moving around, but if ya gotta stay put, you can still make full use of your showmanship.
If you're singing rock tho and have lots of space to move around, like I mentioned having the genre thing into the picture, it helps greatly to move around because of the width of an audience you have.
Maybe if you don't focus on your showmanship, but focus on the audience at every angle, and 'performing' FOR a 'section' one after another and just stop once in a while to communicate certain words in the lyrics or certain climaxing notes, you'll be moving around naturally without even realising it. Then it comes out natural!
Try this;
if you're singing words in the lyrics that require a storytelling / introduction to the theme of the song, communicate it to the audience in sectional parts, like I mentioned above. It's usually, at the very least, during the first verse. Eye contact (you don't have to stare at one person or even see anyone, just look into that area), use your hands if you have to, just do whatever you feel requires of you to 'explain' the song. Good showmanship is part courage / confidence, and part acting. So act/relay the song out, you're kinda like storytelling the mass of people.
For choruses / buildups, focus on yourself a little bit more; pretend you're just feeling the 'conclusion' to it and expose it out emotionally to yourself and your band members, but stay in the middle of the stage and facing the audience as if you're showing that emotion to the world.
Hope the tips helped!
