I play bass in a metal band as well, and have had the same problem to a lesser extent. Since my group is a quintet, I can *afford* to move around a little bit less as our two guitarists and singer really move quite a bit. And I'm not the shyest guy in the world on stage either. BUT...I've been going through the same thing in my head, "What, if anything, can I do to improve my overall live presence?"
Recently, I've observed something that turned into one of those "why didn't I think of this earlier?!" ideas. I study and practice riffs for hours in my personal time, then rehearse with my band 2-3 times a week. However, I spend almost no time on my stage moves.
Two words: Boy Bands (I like to look at extremes to make an example). The best of the best are singing somewhat technical lines requiring tons of breath support and focus to nail...and yet they are simultaneously dancing their butts off. Sure, these guys are supported by the major labels and hire choreographers to help them out, but the facts remain. From what I've gathered, they will spend hours not only rehearsing the singing, but the dance moves as well. Separately at first, and then together.
So how can this apply to your situation? What I've been doing lately is listening to my metal band's music slightly differently. I pretend I am a professional dancer, and it is my job to find a suitable dance for each section in each song. I'll listen to certain sections and realize things like "hey, for these 16 measures I should headbang in a power stance, and then when the transition comes along I'll shake my bass, and then I'll drop close the floor for the final climactic note."
Do that sort of homework and attempt to execute at rehearsals. Then when you hit the stage and the nerves calm down a little bit, what you've worked on will come out naturally with a little luck.
Good luck man! Hope I see you on tour when my band finally gets to Europe!
-Andrew
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EDIT: PS: Keep in mind that different audiences look for different things in a live performance. If you're playing bebop chord changes at breakneck tempos, your not likely playing to people who would get disappointed when you're not moon walking during your solo. As others have mentioned, the same can go for metal. But with that said, I've seen some bands pull off some crazy music while tearing up the stage.