The sharpest change in living standards and technology in human history came about because of the invention of automation - inventing machines to do the work for you markedly increases efficiency. With increased efficiency, more time and resources are available for further innovation, so you need to find a reason why the invention, or the implementation of automation is very unlikely or difficult to achieve.
Limit knowledge sharing
New ideas pretty much always come from previous work done by others. In order for technology development to essentially stagnate, you need to ensure that it is difficult to come across existing knowledge, and difficult to share new knowledge when it is discovered. This would require a restriction to the access of education. If education is rare, it will to some extent self-perpetuate - in order to learn you need a teacher, or educational material (books). If few people have knowledge, fewer people can be teachers. For books to be rare, it must be difficult to manufacture them so don't allow the society to invent the printing press.
Keep population density and population growth low
It isn't the overall population that matters, but the population density, when it comes to knowledge sharing, so find a reason why population density remains low. It could be due to a lack of food resources (see below), or due to diseases restricting population densities that get too large.
Another way to keep population density relatively low is to have population centres far apart. This could happen if resources are scarce, or if the terrain is such that very few places are able to easily increase in size.
Keep farming inefficient
One way to keep population growth limited is to have food production be very inefficient. As long as food production is slow and labour intensive, a large proportion of the population will be required to farm, with very little spare time or money. There could be many reasons why farming is difficult:
It could be natural - a lack of fresh water, or poor soil (possibly due to some ancient catastrophe, possibly because crop rotation hasn't been invented), or excessively rugged terrain, or a high prevalence of disease in food crops, or a high level of pests that eat the crops (e.g. locusts).
It could be societal - e.g. in a feudal system where all the land is owned by wealthy (non-farming) landowners and the farming is done by serfs, there is very little incentive for the serf to come up with ways to improve farming efficiency. They won't benefit, only the landowners will.
It could be due to war - e.g. if an army invades the land, takes all the food and burns all the fields every few years, you're unlikely to be able to produce the surplus of food usually required for population growth.
Scarcity of resources, especially fuel and metals, especially iron
In a world where the only available fuel for combustion is wood from trees, the course of any industrial revolution would be hugely slowed, due to it being much less efficient a fuel compared to fossil fuels. Some of the recent jumps in technology have only been possible due to the ready access to fossil fuels like coal and oil. Additionally, if relying on trees for fuel, you need the space and time to be able to grow the trees.
Similarly, if metals like iron are difficult to get hold of, it's difficult to manufacture the tools required to invent new technologies. Such tools would become very expensive and restricted to only the most wealthy sections of society. Also, if wars were constantly being fought, the powerful may want to use most of the metal in the formation of weapons and armour, rather than tools for the manufacture of goods.
Restrict the movement of people, by making travelling difficult and take a long time
Sharing of new ideas is much more rapid if it possible to easily encounter people from different societies and geographical areas. Making travel difficult and/or take a long time is one way to restrict the movement of people.
Travel could be difficult for political reasons (e.g. warring neighbours), safety reasons (e.g. unpoliced travel routes), or a lack of resources (e.g. not very many roads/not very many transport operators).
Travel could take a long time because of several reasons. Places could be very far apart, transport could be very slow, there may be very few roads, or very poor roads.
Don't have religions
One interesting point that may be worth exploring is having a world without any religion. One of the largest sections of society that had the wealth, time and inclination in medieval (and pre-medieval) societies to travel and spread knowledge were religious institutions. Monasteries were large centres for learning in medieval Europe, especially as centres for reading and writing. It would be interesting to explore the idea of how a lack of religion would have affected the spread of knowledge. It is of course possibly counterweighted by the fact that organised religion can be a large factor in limiting the spread of "heretical" ideas and spreading mistrust among (differently faithed) neighbours.