Let's get this out of the way; most college students don't do all of the "required readings." There may be any number of reasons for this - you were super busy, you decided to actually sleep for once, or you just didn't need to. Quite a lot of readings assigned to college students don't really need to be read, it never hurts to read, but frankly sometimes I don't want to read about ethics in psychological research for the fourth time. Now not reading usually doesn't raise any issues during class but once in a while, there are the dreaded class discussions on that essay you forget existed. Luckily for us, college also teaches us how to bullshit as well. Here is some advise on how to deal with that situation.
Any type of introduction is your friend, if you are reading a research article all you need is the abstract. So if you don't have any time or just forgot try to skim the abstract before class to give yourself a broad overview of the class discussion.
2. Outside Information
Draw in anyone outside information you have on that subject and if you need to talk during class bring that up. "Well in another class," and you get the rest. Also, use real life examples, most professor love that.
3. Open Ended Questions
When in doubt ask some open ended vague question to the class group and relax. Wording is important on these, use some key terms and maybe a few SAT words but don't go to crazy or people will be suspicious.
4. Piggy Back
Did another student come up with an insightful view or good question from actually reading and doing what they are supposed to do? Well, get in on that and use your fellow students good work ethic to your advantage.
5. Ask a Connected Question
Ask a question you have about something that is connected to the issue at hand but not something you would have to read the article to know about. For example, you are talking about a research article about the effectiveness of a certain therapy on people with PTSD you can ask about the exact qualifications for PTSD and how it can differ between patients. (If you can't tell I'm a psychology major).
6. Don't Over/Under Talk
Don't talk way less then you normally do but also don't talk way more then you normally do. Take this class period to fly under the radar. If you are really in a bind you can always polish your acting skills and come in acting like you don't feel well to lower expectations.
7. Don't Lie
If a teacher ever directly asks if you did a reading for class, don't lie they probably already know you didn't do it. Don't give excuses either, everyone is busy. If you do it will most likely just sound like whining. Own up to it; tell them you just forgot or didn't have the energy. Having a good relationship with your professor will make up for being unprepared at least once or twice.