One Inch Square

This tip comes courtesy of my DM, @WolfSamurai who in turn got it from his DM, @DMSamuel, and is a quick and easy way to engage your players.  When a player character kills a monster, instead of describing its untimely end yourself, turn to your players and say, "Tell me how it dies."  Passing off this small bit of narrative to the players empowers them, allowing them to roleplay their hero's triumph, and revel in the gory (or whimsical) details.  You've also offloaded a small part of your already tremendous workload, freeing up some mental bandwidth for more important tasks (like rallying your minions to show those pesky PCs who the king of the dungeon really is).  You will probably want to save the narration of your unfortunate villain's fate for yourself, but your players will likely find being handed some narrative control is rewarding and fun.  Little things like this help engage players, allowing them to feel more immersed in the action and more like active participants in the story rather than passive bystanders.