Here's a tip from Constance Staley's Fifty Ways to Leave Your Lectern, that suggests a way to encourage your students to pay attention to your lectures, as well as help students test their critical thinking skills.
Before you begin your lecture, distribute index cards, and announce to the students that you will intentionally insert a piece of misinformation some time within the class period. The students must "Find the Flaw": listen for the inaccuracy and write it down on their index cards when they think they've heard it. After your lecture you can either, a) ask students to research and correct the flaw before the next class, or b) use the students' answers to fuel a discussion in class. In both cases, you've invited the students to take a more active role in the day's material. -DG
Source: Constance Staly. Fifty Ways to Leave Your Lectern. Belmont, CA: Wadsworth, 2003. 117.