Two pieces of GMing advice I like to give are linked by a common idea: that a single detail can make a surprisingly large difference in creating texture.
Tip #1: You can avoid having your shopkeepers or other random, functional NPCs feel flat by giving them an agenda. This agenda should usually be simple, mundane, and not particularly relevant to what the PCs are doing. While that may sound boring on paper, if you use this agenda as a roleplaying hook, it'll go a long way towards making your NPCs feel like people instead of like interface elements, and in turn help enhance the feeling that there is a world beyond what the PCs see. For example, perhaps a clerk's agenda is that he'd really rather just go home and spend time with his kid than be here. Or a shopkeeper might really want to cook a fancy meal on her upcoming day off, but she's short on ideas on how to pull it off. Of course, occasionally the agenda shouldn't be so simple — sometimes you really do run into a janitor who secretly wishes the younger princess would ascend once the king dies because the older princess shoved him aside during a visit, but can't really act upon it!
Tip #2: When drawing maps of regions, countries, continents, or whole worlds, it's easy to get carried away with the idea that you need to know a lot about every settlement on the map. To my mind, though, if you're putting a dot on the map that you expect to just exist in the background in the medium term or longer, you really only need three details about that settlement: a name, a general size (village, town, or city), and a product. Why a product? Because that not only gives your settlements hooks that you can riff off of if you need to improvise things about them, but it also lets you easily imagine local trade relations. Note that the product does not necessarily have to be concrete like horses, orchards, or iron. It can also be abstract things like trade opportunities (a trading hub), education (a college town), or government (a capital).
Tip #1: You can avoid having your shopkeepers or other random, functional NPCs feel flat by giving them an agenda. This agenda should usually be simple, mundane, and not particularly relevant to what the PCs are doing. While that may sound boring on paper, if you use this agenda as a roleplaying hook, it'll go a long way towards making your NPCs feel like people instead of like interface elements, and in turn help enhance the feeling that there is a world beyond what the PCs see. For example, perhaps a clerk's agenda is that he'd really rather just go home and spend time with his kid than be here. Or a shopkeeper might really want to cook a fancy meal on her upcoming day off, but she's short on ideas on how to pull it off. Of course, occasionally the agenda shouldn't be so simple — sometimes you really do run into a janitor who secretly wishes the younger princess would ascend once the king dies because the older princess shoved him aside during a visit, but can't really act upon it!
Tip #2: When drawing maps of regions, countries, continents, or whole worlds, it's easy to get carried away with the idea that you need to know a lot about every settlement on the map. To my mind, though, if you're putting a dot on the map that you expect to just exist in the background in the medium term or longer, you really only need three details about that settlement: a name, a general size (village, town, or city), and a product. Why a product? Because that not only gives your settlements hooks that you can riff off of if you need to improvise things about them, but it also lets you easily imagine local trade relations. Note that the product does not necessarily have to be concrete like horses, orchards, or iron. It can also be abstract things like trade opportunities (a trading hub), education (a college town), or government (a capital).