I believe you should begin studying for a test a week before it occurs. If you have taken good notes during the classes leading up to the exam you have a good foundation. Often course notes are meant as an addition to, not a replacement for your textbook. Lectures and textbooks often fill in gaps for one another. Pay attention to the cases of overlap between textbooks and lectures, more often than not, it is this subject matter that ends up on exams.
Study groups are also a good idea provided EACH MEMBER DOES THEIR SHARE OF THE WORK! One way to make sure this gets done, is to take the study guides teachers and professors give students and break it down among members of the group then e-mail completed sections to each other. Once you have each other's completed sections (IF THEY ARE NOT BEING GRADED,) compare answers and ask questions. Someone else might have a better understanding of a particular matter than you you do and you them. Do your work and help each other. Everyone will do better in the end.