The best way to keep yourself organized is by using a good agenda or a special tool like Schoolhouse, which acts like a manager for all of your homework.
Not only can you add detailed information about each exercise that you must complete, but you can also use it to supervise the development of more long-term projects, as well as remind you about tests and deadlines that take place on a certain date.
Adding the name of the teacher assigning you the material, a friend or classmate who you're working with, and storing your class notes are some other operations that you can perform with this program.
With Schoolhouse you can also subscribe to Classcast, which is a sort of RSS feed including everything that you need to know about certain classes.
There's more: Schoolhouse even includes a small word processor so that you can work on assignments within the app and attach the resulting files to have everything in one place.
For Mac OS X 10.4 or higher.