Your problem sounds too familiar to me primenumber...When i was at school, i hardly had to make any effort, i didn't even had to study before some of the exams, it was all there, and what i didn't know was easy to get...What didn't stick, will come in a way or another , with some mroe understanding...I didn't concentrate in any of my classes, and didn't do homeworks and not much effort was needed home.
At college, i got through the 1st general year the same way, but on the contrary instead of gettin all A and maybe some B, i got all the kind of gardes from A>D. Which was a kinda drop.
The 1st nuclear engineering year, i got an overall c (a collection fo a, b,c) in the 1st term, and i did it the same way, only relied on my understanding from the lectures, and most importantly, many people ahd trouble understanding, so i used to explain whatever they didn't get, and it was a type of collecting information and studying...But again no effort was done.
The 2nd term, i failed in 3 subjects (cause i also dropped lectures) out of 6 because things got heavier, and i needed to work my hands. And study more. And i really couldn't accept the idea ofworking or doing any effort, for that i dropped 2 years, for 3 subjects in one term.I've no troubles with reading or fetching for what iw ant, just couldn't habituate myself to the idea of "hard work is needed"
After that i started to accommodate myself in the new system, i had no trouble in figuring outhow to do it and here are the tips:
1- The lecture is 100% important, even if you are not really concentrating, it's important, u'll remember a thing or 2 and will help you to better understand the subject, if you tried to train yourself on not gettin distracted and concentrating on the lecture, this will be a super transition for you. That's what got me back to As and Bs...
2- The notes are also quite important, specially that there r some subjects where you cna hardly rely on any books, for example lecture notes were the msot important to me when studying nuclear physics, because all the books available were not satisfying, this will help you not to get lost, and to keep on track with the main focus points of the course.
Also sometimes there are things you wouldn't find in books.
On the other hand a subject like project management is not something to take notes of, i make sure that i concentrate at least 90% of the lecture and then try to solve problems, and this is how it sticks...
So taking notes or not dependign on the course nature, also helps you keeping track of what's important, moreover, i write what i understand not what the professor write or says...
3- Your first referrence is again, the lecture notes, you strart with these, if there's something that is not clear or if you need to know more about it, u'll have to ask your professor for the most recommended textbook.
Sometimes the course depends mainly on a sum of textbooks not only one, a bit from here, another from there, the lecture is what tells what is important, and what do you need from each book, so u start with book one, there's a missing point that's been said in the lecture, u look for another book and so on...
4- generally what keeps the informations in your head, is thinking of them, trying to find applications for them, solving problems, thinking of everything that surrpunds you...Explaining to yourself, why do we calculate it this way,. why did we choose this method and the other not...
In resume, the best way to keep a piece of information in ur hea dand to get a better understanding, u need to use it!
Oh that was really long.