Go for a society relatively stable and isolated, with little social mobility and knowledge being concentrated at the hands of few, and give them a strong cultural/religious basis.
You're basically looking for what we had in medieval Europe, which remained with relatively few major changes during the period. This was due to a series of factors, but the most important might have been isolation. With the constant threat of invasions, everyone was terrified, which led people to group together in societies where money became secondary over survival. Peasants worked to feed everyone, knights and nobles trained to defend everyone and priests prayed and preached to save everyone's souls from hell. This resulted in a society where, safe for things like the Church noble relation (a noble can become part of the Church), there was little to no mobility (a peasant could become a knight which meant going up a class, but that was rare).
So, after this poor explanation of the middle ages, your society would have to be divided in many relatively isolated kingdoms and follow a similar path, in which classes are strongly determined and social mobility is reduced, everyone has specific functions and they are more focused on themselves and their surviving than they are about making business with the outside world. This could have many degrees, likely kingdoms closer to the borders being more open to business than those in the middle of the country, distributing their goods at a higher price even. The reason for such isolation to the outside could be justified by religious or cultural beliefs, maybe a sense of superiority to other countries or even with them being considered heretics, and you'd have the ideal conditions for items from the outside to take long periods to be available to everyone.
Another thing that would help would be assigning to outside products a connection with the higher classes. Maybe because trading goods isn't as predominant in these isolated societies, even the cheapest outside products would be seen as goods that only the higher noble classes and members of the "church" should have. That way, with the culture already established, a peasant wouldn't use noble items, for they'd be likely ridiculed for such "petulance".
Lastly, concentrate the knowledge available to a few groups. One alternative would be to have something like what the Church did, saving many archives from the Greek and Roman eras and keeping them safe, but partially out of reach from those who weren't part of the Church. With the knowledge kept in the hands of few, you'll prevent your society from making their own technological advances, as long as you attend to the last main problem: keeping it from changing.
One of the main reasons the middle ages ended was the end of the invasions, which allowed many structural and political changes. People could leave the fortified castles with less fear of being killed by invaders, cities could grow again, business could become more prevalent, populations could grow more, etc. And it was exactly this series of changes (along with issues such as the plague, the crusades and the hunger caused by population growth and deforestations, but changes nonetheless) that would cause the middle ages to end, giving place to the modern age.
To sum it up, if you try to follow the middle ages pattern, your society needs to remain partially isolated from the outside and have as little changes as possible, so the systems that exist within it don't collapse. Your society can expand its territory, but it must have a reason to minimize exposure to the outside and prevent drastic structural changes to its insides. I'd love to explain in detail how it all worked, but there simply isn't space to do so. I strongly recommend you to research about feudal societies to understand what made them tick and see which traits would you like to use on your own world.
[Note] - sorry, I pretty much remade the answer as the last one had a decent amount of innacurate and partially biased information. I still think something like what we saw with the catholic church (a strong institution with vast power, positioned as the highest elite) would be beneficial to ensuring a society from suffering too many changes too fast, as it has the power and influence to prevent such changes from happen or to at least mitigate them as much as possible.