Here’s something that I wrote on the Nanowrimo forums that I thought was interesting:
Well, I talked at length about the -punk type with friends and I'm on the internet, so surely my opinion is useful here.
The -punk group derives from the original word cyberpunk, which was a term designed to juxtapose. Cyberpunk was about 'high tech, low life,' where the trappings of film noir – grubby, small human interactions, criminal underclass, poverty and a lack of moral clarity – were wedded to futuristic views of technology.
-Punk sort of got watered down after that point, where it generally just means 'modern production using old style.' Consider that Dishonored, a videogame with whale-oil powered tesla coils and not-quite-flintlock guns was being called dieselpunk, Bioshock was called Decopunk and the Eberron campaign setting in D&D is called magepunk.
If you want your punk to come through with your steam, you need two things. You want deliberate anachronism, and you want juxtapositioning. The deliberate anachronism is easy – in an era of history when we didn't have cars, putting in steam-powered cars and an industrial tech revolution can shift things. In our world, these technologicla shifts brought with them social change (super-simple historical bullshit summary I know), which you don't have to have. But it's not the aesthetic of the setting, it's the elements of that setting that our technology would have helped us move past. Since you've said steampunk, you probably want Victorian England as your base. Corsets, goggles, tophats etcetera – but what made those settings interesting beyond the aesthetics? For my tastes, there's the class system, the just-world outlook where the poor were assumed to be dirty and sinful while the nobles were presumed to be nice and just. Find something in the Victorian era you think will make an interesting background element when it's juxtaposed to the story.
Because that's the trick. The Anachronism casts a background to the point you're trying to make. Alan Moore once said Nobody writes about the future, they write about now in disguise, and that' strue of Steampunk too. You can write a story which is set in Victorian England with phlogiston-powered steam-guns and rattling zeppelin sky wars, but you're really writing a story about people. If we keep with the class system idea, what about a character who is technologically gifted, but who has been caught up in a Jingoistic war movement to invade Poorer Country for its Phlogiston reserves?
Think noir, think contrasts. That's where good Steampunk (or good anything-punk) come from. Steam tells you what well you're delving into for your contrasts, and punk reminds you that the story is about people.