I think by far the most important thing is to have a genuine interest in the subject matter. If a person is studying not from real interest, but just to get a job after graduation, this in itself is a huge barrier to learning.
I had a very difficult time with organic chemistry, until I tried a new way of studying. Every day, I studied about 3 - 5 pages ahead of where the class currently was. This means I learned material on Tuesday that was covered in class the following day, on Wednesday. I read/studied about 3 - 5 pages AHEAD every day for about 30 minutes. This way I found the professor's lectures much more useful and clear. It gave me an advanced look and I knew what questions I should ask.
Another piece of advice is to ask a lot of questions, both in class and in your professor's office. Professors like students who are interested in their subject, because most students are not.
