Commit to studying at least an hour out of your day. It isn't a lot given that you should be studying three hours a week for every three hour course taken. Carve out some time during the day to commit to studying your hardest subject. If you have more than one, then you may need to commit to an hour and a half.
Read your assignments. One of the biggest mistakes of college students is the tendency to not read their assignments. Look over your syllabus and ensure that you keep up on deadlines for papers, tests and projects.
Prioritize your other activities to go behind your academic requirements. It can difficult for students who are involved in extracurriculars, but it can be managed. Make to do lists in which you rate your obligations in order of importance. Number one on your list should always be your class work.
Attend class regularly, since its will be the biggest determination of whether you receive an "A" in the class. Note that most professors come to know students who frequent their classes. They will be a bit more lenient on curving final grades for students who are committed to attending their lectures.
Join a study group or academic club, especially in the classes that you struggle with. This group will enhance your sense of understanding and make learning the material a little more fun. Consider getting a tutor to help you learn the concepts that you have problems grasping.