There are only two types of NPC's: those that are forgotten and those that are remembered. Sadly, most storytellers have forgettable NPC's. That is a shame because with a little work (and not a lot truly) you can create 1 - 3 NPC's that the players could love or absolutely hate.
This brings your game to life. Now I am not advocating that EVERY. SINGLE. NPC. should be remembered, but the ones that are helpful to your players should be. The ones that are recurring should be. The ones that you might want to kill much later for dramatic effect have to be.
Listen to find tips and tricks from using different voices or accents to props if you can't modulate your voice to make your NPC's stand out.