Alternating topics or types of questions. By alternating during learning, you continuously engage your brain, thereby strengthening the connections between the neurons in your brain. This makes it easier to retrieve information from your long-term memory (see forgetting curve). If you keep making the same type of questions about something, you eventually work on autopilot and don't have to think anymore (and thus you don't learn). Ways to engage in interleaving include making random practice questions (test questions, quiz questions) from material covered in the past weeks (see retrieval practice). By repeating this, spaced over increasing amounts of time (see spaced practice), the information will stick better. However, it is first and foremost important that as a student you thoroughly understand the material you are going to learn. It is also important that the topics are related to each other. For example, it works well to mix exercises about the volumes of a cube, sphere, and cylinder, but not to mix different subjects. This way of learning will feel more difficult and therefore less enjoyable than simply studying the same thing for a longer period. However, this actually means that your brain has to work, and as a result, you learn more.
Rohrer, D. (2012). Interleaving helps students distinguish among similar concepts. Educational Psychology Review, 24, 355-367.