Alternate different topics or types of questions while learning. This variation forces you to actively use your brain to think: if you keep answering the same type of questions about something, you will eventually work on autopilot and no longer need to think (and thus you do not learn). Test yourself by answering different types of practice questions about the learning material or by writing down the definitions of concepts from different chapters from memory. Make sure you first understand all the learning material you are going to learn. It is also important that it is about the same topic: for example, do not study different subjects at the same time. This way of learning will feel more difficult, and therefore less enjoyable, than just studying the same thing for a longer period, but it means that your brain has to work and that is learning. By repeating this over and over, spread over more time (see spread your learning moments), the learning material will stick better.
Rohrer, D. (2012). Interleaving helps students distinguish among similar concepts. Educational Psychology Review, 24, 355-367.