As i was ordering a Chai Latte at Birch coffee at the Gershwin Hotel, I noticed that they had, not one, but two Tip Jars that were completely full! The trick is to engage the customers by asking them a simple question with only two possible answers. The customers are engaged and are more tempted to tip by putting money in the jar that they think holds the right answer. The waiters end up getting more dough and follow up the next day by writing the right answer on the board.
Great way to keep your curious customers coming in and for creating a loyal consumer base.