If you want to take a lot of courses/units in a short period- like summer session- make sure you do the following without exception:
-Sleep 8 hours a night or more EVERYNIGHT, even the nights before an exam. You can retain and maintain a stable understanding of material if you get 8 hours of sleep a night.
-Eat healthy (no junk food, it can slow you down during the day when you really need to focus)
-Drink a lot of water throughout the day. I know this seems simple, but a lot of people don't realize the ill effects of dehydration especially during the summer time. This can give you the much needed edge to successfully preform well.
-Do an hour of physical activity everyday.
-Sit in the front and take VERY detailed notes. If you can, record the lecture as backup in case you didn't write something important, especially if the professor said it quickly. Keep in mind that you shouldn't listen to the entire lecture again because you won't have time.
-Rewrite your notes (on the slides or on a blank sheet of paper) right after class. Make sure it's organized and that you have room for extra notes from the book if needed. Notice: Try to figure out if the book is absolutely needed for the course. Having to read the book/take notes on it is a beast of it's own. It's always easier when you don't have to deal with the book. I usually don't buy the book or use it unless the professor specifically tells us that they will test us on it or if there are homework problems assigned from the book.
-Look over all your notes as often as possible. Essentially, you want to look at lecture three times in the week. No more or less than that. Looking over your notes weekly is KEY to doing well on the midterm and even on finals week, especially if your taking 3-4 finals. You need to make sure that you completely understand the material that you're learning. By going over it a couple of times, the material will eventually sink into your head. It's really hard to describe, but if you have ever used this technique, you will understand what I'm talking about. It's going to be hard to go over ALL of your notes three times a week, especially when you have to learn more stuff that day, but trust me, this is gold. Don't spend more than 15-20 mins per lecture. Try to do this review thing as soon as you wake up. For some reason, I retain more information in general if I look at it right when I wake up.
-TALK to someone about the topic or rather, teach someone the lecture. This is the best way to learn something. Try to make a friend in each of your classes and talk to each other about the topics/concepts. Real life example: I carpooled with a friend of mine who was in the same biology class, and the only time I studied or looked at my notes was during our 30 min carpool to class everyday. I got an A in the class by doing that alone.
-Find out how your professor tests, such as if they focus on more lecture material or book material. Ask people who have scored well or who have not scored well in that very class how they studied. Learn from their mistakes. This is a study "smart" strategy.
You need to take one lecture at a time and try to focus on what is in front of you. Make a to-do list the night before and make it your goal to finish everything on it. Even things not academic.
DON'T count how many hours you're studying. It will only hinder your focus. Just try to get everything on your to-do list taken care of regardless of the time it takes. Of course don't over do your to-do list, but you know what I mean. Sometimes you can study for hours without realizing it. That's the best way to study long hours, because everything goes faster and smoother when you're not constantly timing yourself.
Make sure you take breaks often during your studying. Don't exhaust yourself. The amount of time you give yourself a break could be 10 mins or even a walk to the nearest restroom. It's got to be whatever your comfortable with and enough time to allow you to stay focused for another hour and thirty minutes or more before taking another break.
Study in an area that is new to you and that is away from distractions. However, make sure you are around other people studying. This will keep you focused longer. Try not to sit next to friends when your studying long term.
Turn off your phone, don't check facebook or any other site during your breaks. You want to clear your mind during the breaks, NOT get distracted.
Try not to eat during your studying. Eat during your breaks.
-Take a power-nap, no more than 10 mins after lunch. This will give you an amazing boost and will allow you to retain information that you learned in the beginning of your day.
Definitely take a day off if you can, but I suggest that the day off should only be review of old material/lectures.
If you can follow this advice, I'm sure you can pull out all A's in all of your classes, no matter how many units you take or which semester/quarter you take them in.
These are the strategies I have learned and used during my past three years in college, and I have done very well because of it.
Good luck.
