Make them look functional
If you add gears make them look functional. In fact, make everything you can look functional rather than decorative.
For example, I’m planning a rather large steampunk robot. I plan to add something that will make it look like pistons power the wheels. I’ll probably use plastic plumbing pipes for the cylinders, and hook up the pistons to the back of the wheels. Yes, the wheels will power the pistons, but at least it will look like the pistons power the wheels.
There will be gauges in various places, and a few screw down valves.
The paint job will be for the most part in black and white as if over forged iron. There are a couple of robots here with the look I want, and they got it with a very thick layer of silver spay paint, and then a thing layer of black over top. That should give it a textured look. I’ll have to experiment on some pieces of scrap wood to get it to look right.
Another suggestion is to make it look like a firebox. Have a small grated opening and have a couple of LEDs there to simulate the look of a fire. There should be something in YouTube where you should be able to see a steam engine in detail.
You can also make it look like rivets are holding it together. I’ll be doing this by creating such things with my 3d printer.
LEDs are great for a lot of effects. I’ll be surrounding both “eyes” with AdaFruit NeoPixel rings so I can put whatever background color I like on the eyes.
My robot is designed to look sort of humanoid. It will obviously be a robot, but it will balance on the two bottom wheels and be about five foot tall. When I get the money I’ll put arms on it, but they won’t be able to lift much on my budget. The end point is a butler bot based on Groucho Marx as a butler.
If you have weaponry, try to make it look old fashioned, such as Gatling guns rather than lasers. There are a couple of Nerf guns that could be changed to have a steampunk look.
Make up a story with exciting names. For example “This robot is the first prototype of Professor Doomboggles giant army of steam powered penguins.” sounds much better to me to me than “I just made this for an art project.”
If possible, try to make the wheels look like something out of the Victorian era, rather than just using skate wheels.
I like Duane’s idea of using copper tape. I would, however, practice putting the tape on your base material so it looks natural. You might even be able to get thin brass sheet at a hobby store and make the sides and top of your chassis out of that.
One place you can look for steampunk ideas is instructables,com. Just search for “steampunk” there.