Get tutoring. For many students, sitting through the class isn't enough. Your teacher's explanation of a math concept, as good as it might have been, could have failed to sink in. Tutoring ensures that a student has an accessible explanation of skill sets.
Take advantage of practice tests. When you go through a handout assigned by the teacher, you are working out sample problems that will have a lot of relevance on test day. A practice test is generally a good survey of what you've covered and helps you anticipate what kind of problems will be on the test.
Identify weak areas with self-diagnostics. Find resources for self-testing, whether through a paid service or from your school, and do diagnostic exams to see what types of problems you need to focus in on. This is a powerful tool for fine-tuning your skills before a big test.
Learn all applicable chapters. Even if a teacher drops your lowest grade, you still need to know all of the math involved. It doesn't help you out a whole lot to get really good at a few things and zone on some others. The balancing grades might not guarantee a passing one. But if you can confidently predict a decent C grade in each chapter test, you should be ok.
Check in with classmates. If you know someone who is coasting along in the class, plan a study date. You'll be able to assess how the other person is passing the class and boost your own chances.