Advertising is a really big part of real world city environments, and having ads in a game (fake or otherwise) helps to bridge the subconsious gap between real world and game world. In da_trainingday I started to make some posters to put around the trains station - just go to any subway station in a modern city and you'll see that 90% of the signage is advertising. I only got as far as this one before moving onto more pressing matters:
People don't often actually notice what is written on the poster. They see a colourful rectangle on the wall and assume it's a poster, then continue shooting people. This is the little psychological nook and/or cranny that I like to play with. I like to poke fun at stuff where I can, and "Nothing but glucose" is a stab at soda, which is pretty much carbonated glucose with a splash of water. You don't need to poke fun, and you should be aware that whatever you put on these posters is going to be skimmed over and largely missed by the majority of players. That means that we should reward the few players who actually do take notice with something cool or humourous. At best any message will be subconciously embedded into the less observant players minds - which is an awesome place to be with regards to content writing.
So what do you put in a poster?
Really you can put anything you want in there as long as it's PG 13, but don't be afraid to walk the line. If you want more guidance, read on.
Things like fashion, make up, food, restaurants, hotels, travel, movies and TV shows make up the bulk of advertising. Create a brand of one of these things (a name, word or a business) and make a logo. Put that logo somewhere on a poster-shaped canvas and then give it a background. Try to put extra stuff in the background - I used a radial gradiant and some bokeh stuff I had laying around on that green poster. Also think about grunge overlays, reflections and other details. Then put an image depicting the item you are trying to sell somewhere on the page too. Move all that stuffa round till you have something that looks nice. Finally, add some small print. Most posters have small print and putting it in there helps again to bridge that psychological gap.
Using references is HIGHLY recommended. Try to keep the colours simple and clear - I'll be casting lights from the posters to add colour to the environments and if you use a lot of different clashing colours the light color will be muddy and kinda useless. Try to stick to one main colour temperature
and vary it for detail - you could use a deep blue that graduates to a bright azure, for instance, with a complimentary colour for details.
Also try to work slogans into the posters. Here's a few I'm making up as I go:
Travel: "Meet people from all over the world! And then shoot them."
Make up: "Because you're not beautiful until we say you are"
Food: "Cockroaches are protein, right?"
Hotels: "Bedbugs stay free"
TV: "Your friends suck, use ours"
I'm all out, go crazy!
EDIT: you could also advertise video games (since we're in one) and guns (since it's a gunny kinda game)