A completely different approach to school to start with; in my undergrad, the motto "B's and C's get degrees" sufficed for me... that's besides the point though, but I coasted through each course with minimal effort, settling for just above a B average. When it came to my post-bacc, I began each course with a thorough understanding of the syllabus and grading structure. I knew where I would collect the easy points (chem & physics homework, labs, etc) and I would ensure that those point columns would be near 100%. Don't give up any easy points!
Second, reading... I didn't do so much reading in my undergrad, which certainly affected my understanding, and the bottom line grades. I now try and read before every class. I go into lecture waaay ahead, having either a parallel or alternative perspective of the lecture. This has been huge. Learning is way more fun when you know what's going on, not lost in the weeds wondering when this stuff is going to come back together and make sense. You are able to be way more engaged and on top of each concept. I also found that I get a much higher yield in terms of understanding and retention and when it comes to reviewing the reading after the lecture or pre-test, you can skim through and focus on the concepts that are still a bit fuzzy. Way more efficient.
Test prep; practice... Probably the biggest change to my strategy is the way I approach tests now. I do as much practice testing as I can fit in and much less time reviewing notes. I do skim back over key concepts and illustrations from the book, but honestly, the biggest bang for the buck is going over practice questions and truly reviewing the broader concepts (not just the answer) for any questions you get wrong or even guessed correctly on. Remember here though, and this is critical to approaching every test down the road, is to put in the time to re-learn the concept for any questions that you don't nail. Don't just focus on getting the correct answer to that question. I can't stress this enough...
Some intangibles that seem to help is starting off the course by setting up a relationship with the instructor.. An email introducing yourself, your academic goals, etc, have seemed to help keep me on my toes and accountable for acing their course. Of course, in this there's the benefit of establishing relationships with professors who can act as letter writers, who have seen me at my academic potential; an important consideration when going through your post-bacc.