Throughout your academic career, you will encounter courses or seminars with long lectures. While you may dread them; and I know you do, we all have to get through them. Recently I sat through a three and half hour lecture. I have sat through several of these lectures in the past and have put together a few things that work for me, and will hopefully work for you.
Prep in advance
Take some time to go over the material to be discussed in the lecture before it starts. Yes, everyone tells you to do this, but it I have actually found it to be very helpful. Whether you are reviewing your textbook, previous class notes, or researching related topics, reviewing will make it easier to follow along, especially when your attention span begins to drift.
Snacks will keep you focused because you will not become distracted by food. Make sure to balance your healthy snacks with your sweets to prevent creating poor habits. Plus, having healthy snacks motivates me to work harder to focus in lecture.
This varies from person to person, but I strongly suggest consuming something that will keep you awake during the lecture. I usually have a coffee or tea, followed by water.
To follow up on the last tip you must drink water. Staying hydrated is key for your health and will help counter the caffeine in your system.
When I am in a long lecture I like to take more notes than usual. I also make sure to do little doodles and marginal notes. This is very important because you are taking in a great deal of information at once, and while you may think you understand it you want to make sure you understand using it. Make sure to go over the material after the lecture as soon as possible so that you can remember it and fill in any blanks you left out during lecture.
If you are confused ask for help. This can mean asking a neighbor, the lecturer, or writing the question in the margins. This is probably the last thing you want to do when you are tired, but trust me this helps. It will keep you focused and attentive, and fully understand what you have learned.