I have a secret way to learn history.
History is important - maybe your history teacher told you that when you were back in school, learning lists of dates and names and wondering "Why, oh why, am I doing this???" When I was learning history in school, I was motivated by my desire to do well and to know what I needed to know. But there was another way I learned history. I learned it by going places.
I went to France. I saw old stones, old cathedrals. I went to a museum about lacemaking that forever changed the way I see those dresses that the pre-Revolution nobility used to wear. They were wearing months and years of women's whole lives.
I went to Washington, D.C. I looked at the Capitol, and the monuments, and saw what this nation considers important. I went to Mount Vernon and discovered how George Washington lived, and also how his whole lifestyle depended on the work of slaves.
I know I'm lucky to have visited these places. Not everyone has opportunities like this. But doing this has given me an idea about History that I never got in school.
History is present all around us, right now.
When I'm in France, or on the East Coast of the US, I can feel the depth of history around me in the buildings that preserve those old times. I can see buildings that have housed family after family for hundreds of years. When I'm in California I see less of it, but there's still some there. The Missions are there. Even more enduring than the buildings, though, are the names. Malibu, from Ventureño, for example. Or Aptos, the town right near where I grew up. The names give evidence of the deeper history that wars, cultural assimilation and "progress" sensibilities have tried to erase. And if you look closer, and escape the beaten path, you can find a lot more than you bargain for. Like the Welsh cemetery at Black Diamond Mine, or the Chinese town of Locke on the Sacramento River delta.
The worlds you create need to wear their own history.
How old are the oldest buildings you see? How old are the newest? There's a reason why Hiccup in How to Train Your Dragon tells us that the village is old but all the buildings are new. It tells us something vital about the place. Did all the building happen during one short period of time, or is it ongoing? Do old buildings have value? Are they preserved (Japanese Buddhist temples for example), rebuilt in the same form (Shinto shrines, e.g.), or razed and replaced with something else?
What kinds of names do the places have here? Was there an ancient history of people who spoke other languages? Do we see a dead language (like Latin) underlying things, or the language of a displaced people (like Aztecs)?
What conflicts have there been in these people's history? Were there wars and revolutions that changed everyone's lives? How did those changes happen? Are there still traces of the conflict remaining in old traditional enmities, or cultural differences between different areas (like how the Civil War's footprints remain in the Northern and Southern states)?
One of the things I always liked about Middle Earth was the way Tolkien left ruins around, deliberately, not just "Oh, look, here are some picturesque ruins!" but the kind of ruins that gave you a sense that there was a previous history to this land. A long history, that if you wanted to, you could find and track.
If you plant historical things, put them there for a reason. Multiple layers is fine, but inconsistency is not.
Also, build history into your cultures. History builds some groups of people up, and it casts some down, but even those who lose a lot don't lose everything. Here's an example to give you a sense of what I mean. When I was creating the Varin caste system, I knew that the system itself had been created around 400 years before story time, and the previous history erased maybe 70 years after that. But I also knew that there were some groups whose solidarity existed before the caste system was overlaid upon them. One of those groups was the personal servants to royalty, who became the personal servants to nobility when the noble caste was created. The symbol of their identity endured across that change, but took a different form, going from a mark on the clothing to a mark on the person (tattoo). The idea of secrecy was core to their identity for much longer than 400 years, and it affects everything about the way they think, speak, and behave. The other group was the one who became the undercaste. Part of the reason (though not all) why they were cast down was because they had a different religion from the rest of the population. The trappings of their previous status were erased, but their religion endured, along with its imagery, its beliefs, and its sense of tight solidarity. Ask a random Varini on the street what fire makes them think of, and they will say "teeth" or "punishment" (or possibly "the surface"). Ask an undercaste member, and they will say "paradise."
If you don't plan your history, it may plan itself for you. Your subconscious will often layer an idea of a community's history into its depiction without you even realizing it. It's worth taking a deliberate look at a certain point in the process, to make sure that you haven't inadvertently layered in an inconsistent history, or worse, an unwanted piece of Earth history.
It's something to think about.