Bring your book(s).
Make sure you don't forget whatever book(s) you're working on in a particular class! Whether it's a novel (literature majors represent) or a textbook, be sure to bring it with you to class. Follow along whenever your professor references a certain page, and underline or bookmark any passages read aloud or discussed in class. Doing this will not only keep you engaged in class, it'll help you with studying for exams down the road.
While some people claim they don't need to take notes, I believe note-taking is really beneficial because it forces you to pay attention to the lecture or discussion. Write down everything the professor writes on the board, any definitions he gives, and any other important facts and details that he says. Develop a system of note-taking that works for you (whether that's handwriting or typing, bullet points or paragraphs, etc) and use it every class.
Come to class fed so that you're not starving all through class, and so that you have more energy. This is especially important if your class is just before or during a meal time. So make sure you eat breakfast before that 8:00am so you're not dragging and falling asleep, and get lunch before a midday class. Then your stomach will be full and happy, you'll be more energized, and your mind will be better able to receive and process information during class.
Make sure you've had enough water in your day leading up to class, and also bring a water bottle with you to class. Being hydrated will keep your energy levels up and prevent you from feeling sluggish, so you'll be more alert in class.
Put your phone away.
Most professors have a rule against phones in class anyways, but I'm still throwing this one in here because secretly playing with your phone under the table is a real thing. Nothing will take your mind off of what's going on in class more than your phone. Put it on silent, and either put it out in front of you on the table (where your professor can see it and where it'd be totally obvious if you were to grab it), or put it in your bag.
Ask and answer questions in class, or offer comments. Or if you're like me and you're not comfortable talking in class, that's totally okay, as long as you participate silently. Whether you're a verbal class participator or not, just be sure to be engaged. Actively take notes and follow along in the book. Always be looking either at the professor / the board, or the book / your notes. Participating in these ways is sure to keep you engaged in class.
What helps you stay focused and engaged in class?