Ask questions if you don't understand something. Just because others aren't asking questions doesn't mean they understand everything being said. If something isn't clear to you it may not be clear to your classmates. Summarize, re-write, or otherwise review your lecture and class notes between classes. Don't wait for the night before an exam to re-familiarize yourself with the material covered.
Mental preparation is similar to physical exercise.
A little bit every day will make you stronger gradually. A strenuous workout after a long layoff will only leave you sore. There's no reason you can't read ahead. Take some action to personalize the material. Develop your own set of reading notes, summarize each reading, or write out what you think would be a likely essay question.