Take an object in your house; preferably a large, heavy one. Fill an opaque bag with sand so that it weighs a healthy amount. Then at the beginning of your session, reach down and pick it up, and drop it on the game table.
"This," you'll tell your players, "It 10,000 gold pieces." It can be 50,000 or 200,000, depending on the level of your players.
Next say, "This money, and the experience that goes with it, will be given to any player here that's LIED TO by an NPC. If any NPC you meet in today's running lies to you, because I've made them, and the player can provide proof that the NPC has lied, then you get the gold and an equal share of experience to go with it."
This, whether or not you give X.P. for gold. Make an exception.
Finally, say, "This isn't a game of 'spot the liar.' As your DM, I'm not going to give away any of this money tonight. Because none of my NPCs is going to lie. At all. Understand? This is a sign of my commitment that what you hear is the exact truth. Every time."
Tell me what response you get.
