After class, scan your notes. Recall every important things happen in class, the discussions, etc. After that, when you're not sure about something, when you're not sure that you understood something very well, try to browse the internet, that's the proper use of technology anyway, use it in sensible things. Search some quizzes that the topic discussed was involved. Search, read, think, and analyze everything. Try to play some 'games' that will take your interest, but of course, still, the topic must be involved.