If you don't want too many greeblies, but still something that looks functional, perhaps a couple of small knob/button type things near the "breather" part of the mask, that look like they control certain functions of the mask.
I don't have any specific examples, but that could look pretty cool.
In terms of making it look more evil and Star Wars-y I would perhaps suggest repainting it in either a black or deep red basecoat, and then adding some accent colour in the cheek and nose divets, and around the breather vents, or if you can get them to fit well enough, LEDs in the vents would look crazy cool, either in a red or perhaps a very bright blue would look cool. (I personally think the blue would contrast well if you painted the mask a dark colour)
In terms of *how* to do this, well, I am terrible at actually doing stuff like this, so I can't really help there.