Consider this a part 2 of my Paranoia Kabuki Theater post, but it really is it's own topic.
One of the biggest issues I have in an RPG is the bad guys don't look good. Or more accurately they don't look interesting. Lets take the Stormtrooper and Darth Vader vs your typical Orc and Lich.
The Storm Troopers have an iconic white, almost ghoulish armor that you know means trouble. The "space skull" motif really sales the idea that they are both minions and are out to kill you. They are the Empire's version of the Space Marine. Hell even 40K universe gets this. Darth Vader is iconic as well, the helmet (hair), the overly large eyes, the breather mask. His whole outfit radiates authority and dread. If he shows up, you run. Or die shortly there after. It gets worse when you combine the two. The first scene from New Hope during the border...ya. Vader is strutting in with his minions. He looks every part mastermind of this operation. You know he means business.
Other super foes get this look too. Doctor Doom, Darkseid, The Shredder. The list goes on. Each of them stand out, they have a defined LOOK. It doesn't have to be armor, but that's classic. It could be a fancy set of robes, some fetish or jewelry, the point is they have something about them that draws the eye. When a player see this item they immediately know who they are dealing with. This helps establish the mood and makes the NPC more memorable.
Mooks generally get a uniform. Even if it's not an official one. This helps establish either class, role, or ideology of who they are facing. Rangers of the Doomwood might all have Light Grey Cloaks and raven's feathers in their hair. Boom, instant recognition. And you cut down the amount of work you need ot do to help players remember NPCs. Imperial Guard in 40K all look the same, most warriors have the iconic 40K skulls and holy script on their armor. You remember it.
I have a rule of thumb on this. I call it the 4,3,2,1 method.
Major NPCs get 4 descriptive lines. One of which is usually environmental. ie. Darth Vader - Dark Armor, shiny bug eyed helmet, constant his of respirator and deep metallic voice, aura of menace.
Or for another example: Admiral Nex - Bright blue uniform, scared white right eye, sword cane always tightly held in his hands, constant his of sea around him.
Mid-bosses, or major NPC foes or allies get 3 lines.
Minions, or minor allies get 2 lines.
And nobody NPCs get 1. And I follow that. it could be "Creepy" kid, or "Lumbering Ox" farmer, etc. That way if the deal with that NPC again, you just use the one word description so they can mentally find them again.
It's basically a memory trick, and boy does it work.
And heck. Dress them up a little!