I know what your thinking, who wants to be a slave to fashion. The fact of the matter is that although you may not think so, clothes do make the trooper. Some Overlords have taken to a more loosely held fashion sense. Pinhead from the Hellraiser series comes to mind. He and the Cenobites are all different and yet seem to match well.
You don’t need to hire a fashion designer to make yourself stand out from the crowd though. Just be different and portray a sense of foreboding in your enemies and you are well on your way. Your troopers should be comfortable with what they are wearing so simply giving them a set of guidelines with a few examples might be just the thing you need to do.
Avoid cheap knock offs. Stormtroopers are done, the slotted eye holes is the very first no no on the Evil Overlords tips. Roman foot-soldiers, overrated, although carrying a sword is a good thing. Mongol hordes, I think not. What a waste they turned out to be. Same can be said for Para military troops, they all look the same except for whatever flag they have emblazoned on them. Dare to be different and stand out.