Learn from your mistakes. That's the most important thing to do when trying to improve. For subjects like Maths, when you get an exam/assessment back, look for areas in which you made mistakes (such as incorrect calculations/working out, wrong formulas etc). For other subjects (e.g. Business Studies), you might lose marks not for making a mistake, but rather lose marks for not including enough detail in your response. For example, in the business report section, you are required to have a thorough knowledge of the concepts and be able to expand on them and relate your answer back to a given stimulus. Regularly seeking teacher feedback is a very helpful way of ensuring such mistakes don't happen twice.
The problem could also be fixed by revising your study method. You may find that you are not studying enough and this might cause your marks to decrease. It is also possible that your study method may not effective altogether. In this case, you'd need a fundamental rethink of your study strategy. So yeah, these are some things to consider.