This Is I Think, My Best Prep Tip As A DM:

This is I think, my best prep tip as a DM:

When the players are about to visit a new town, pre-generate several NPCs who fit the demographics of the town, but don't give them jobs. Your town is Mostly human, with a number of halflings and gnomes? Make a list that's mostly humans with some halflings and gnomes mixed in, with names that match the vibe you're going for and maybe the barest description + a quirk of some sort.

So the list would look something like this:

Ophelia Bracegurdle, older Halfling woman who laughs a lot

Norabecka Johnson, a young human woman who seems tired

Geraldofinio Babblecock Nimsy, gnome gentleman who takes pains to maintain a fabulous mustache

Etc.

Then, when the players are like, "Can I go to the blacksmith?" You look at your list of NPCs and the one at the top is Ophelia Bracegurdle. She's your blacksmith now. Then they want to go to the tavern, where Norabecka is the innkeeper and Geraldofinio is a patron having a drink at the bar. He's using a straw so he doesn't mess up his mustache.

If they had gone to the inn first, Ophelia would have been the innkeeper with Norabecka as the patron, and then Geraldofinio should have been a blacksmith with some sort of mustache guard to keep the sparks off.

Making the list ahead of time doesn't take much time, and you can often re-use the people you never got to at the next town.

Your world will seem vibrant and interesting and like you have everything planned out.

Have fun!