As a GM, you can show your appreciation for good roleplaying from your players... by showing them some of your own!

A tip to make it easier is to make sure you define the personalities and conversational styles of all your significant NPCs. Get into their mindsets, understand their motivations, goals, and quirks of personality. And then play those roles, jumping back and forth between them in separate compartments in your mind.

An exercise you can try is to think of two very different characters from a TV show or movie you know very well, and pretend to have a conversation between them. Make each one distinctly like their on-screen personality, and switch from one to the other in a kind of verbal table tennis. When you can hold a convincing conversation between two different people - entirely by yourself - then you're ready for some serious and hard core GM showing off in front of your players.

Transcript

Chirpa: That was an exciting seminar.
Paploo: Yes. But he didn't tell us anything about selling services.
Chirpa: Amazing. The Great Reader has demonstrated his superior marketing skills once again.
Paploo: But what about vendor trust?
Chirpa: How can you not trust someone who embodies our philosophy so completely?
Paploo: He's swindled us successfully three times in half a day!
Chirpa: Exactly!