I mastered a challenging subject by making myself fall in love with that subject. By falling in love with the subject, you will seek to find ways during your day to get with that subject, to read and study into that subject, to work problems or write definitions.
This is because by nature we avoid what we dislike. So first off, a student must adjust his mental thinking from "hating the subject" to "really liking the subject" by just realizing that he needs more information about the subject which he can get by studying more into that subject.
Also, appreciate the value of learning new material, whether it seems important to you or not, because all new learning expands the brain, at the very least.
So if you are required to take a class you're not all that interested in, get yourself interested. Once you are "into it" you won't be able to keep yourself away from it.