A couple of nights ago, Mike from Campaign Mastery posted "Wisdom of the Ages." It has some very cool ideas on inserting folk wisdom into your games. The gist of it is that you take existing folk wisdom, like "never look a gift horse in the mouth," make any necessary adjustments to your game's terminology, give it an origin, and drop it in to your world. It doesn't have to always be true, but more often than not, should be. The post talked about building an adventure around it, but I think it would be a great way to insert some culture into world building.
This wouldn't have to be limited to taking existing folk wisdom, though that would be the easiest way to get the ball rolling. This would be an easy way to show players that two nations distrust each other. For instance, in Kirinioth, Dwarves and Elves were originally created to serve the Oreads and Dryads, who in turn were created to serve Aengeinhaur and Veaengeihvean. Eventually, they both sided with the humans and their allies, but the distrust has managed to linger in the thousand-plus years since. By giving the other races phrases and reasons (real or imagined) to show their distrust, it emphasizes that helps to add character to the otherwise flat stat blocks populating the world, without having to develop a personality for each individual person the players interact with.
Something like "never turn your back on a salad with pointy ears" could be a phrase to show that humans feel that elves have betrayed them in the past. Bear with me, it's 1 am, but as bad/odd an example as this is, it does immediately tell you that someone doesn't trust elves, and that they don't consider them to be equals.
I'll aim to work on a few phrases that can be given to some of the people of the setting, along with the other details I had in mind, like popular clothing colors, types of foods available, etc.
I should probably add something here.