Give your characters life

Name your NPCs. Give them distinct personalities. Wants, needs, and quirks. Opinions and beliefs. If you can manage it, portray them with distinct voices or mannerisms. Bring them to life and play them as if they were real people.

Describe your NPCs with memorable traits. Players often have a hard time remembering that the smith is named Taliesen, but they'll remember that the smith is a big loud guy with a shock of white hair.

Don't let PCs walk all over your NPCs. Be a fan of the player characters, sure, but that doesn't mean you should have your NPCs just do whatever the PCs want. If it feels like the PCs are pressuring or enticing your NPCs into doing something that they wouldn't want to do, well, that's what the Persuade move is for.

Give your non-human characters life, too. Describe how they look, of course, but also how they move, what they sound and smell like, how they act.

Beasts, spirits, even the utterly alien Things Below—they've all got personalities. Develop a sense of their motives and how they act under pressure. For beasts, think about their ecologies. For the undead, what keeps them from passing through the Last Door? For magical entities, think about the strange rules they follow. For all of the above: put those details on screen, or at least hint at them. It goes a long way towards portraying a rich and mysterious world.