First, and foremost, make sure you make it to each and every class throughout the semester. You will need to take good notes and pay careful attention to important concepts the teacher emphasizes.
Try to obtain an outline from a 2L or 3L that took constitutional law with the same teacher you have. Make sure the person you obtain the outlines from are trustworthy and not out of date (though this is rare for Con Law classes). If you can't obtain an outline from a friend, then consider obtaining a commercial law school outline.
About a month before your exam, begin creating an outline from your notes and other sources. This is where a good outline or hornbook comes into play to explain holes you may have in your notes. Start making notecards as well if that is helpful to you. For example, label one side of the notecard "Marbury v. Madison" and then place the main ruling on the other side of the notecard. Use these notecards to study daily.
About two weeks before your final exam, start studying vigorously for the exam. Allocate an hour or two a day to study strictly for Con Law. Also, make sure you don't fall behind in your reading for that class and other classes.