Notes. Teachers always want you to make notes, but do you really know how you should make them? Do you write down everything? Do you only seem to write down the first 5 minutes of the lesson? Maybe you use a laptop? When you look at them are they useful?
There's a lot of theory behind note-taking. Recently, there's been some controversy over the use of laptops for taking notes and whether they're actually worse than writing notes by hand. Whatever the truth, the fact is that notes are a necessary evil. The crucial thing is that they work for you. however, you make them, remember that it's only important that you understand their content. Personalise them as much as possible. Use diagrams, abbreviations, the colours that you want. Anything that makes them memorable to you.