The first method you can use is called chunking or clustering. For instance, if you needed to memorize which hormones are released from the anterior pituitary gland, you can use the acronym (courtesy of Kaplan MCAT Biology): FLAT PEG (FSH, LH, ACTH, TSH, and Prolactin, Endorphins, GH). Separating the acronym into two words allows you to remember that the hormones within "FLAT" are tropic (i.e. act on another gland) and that the hormones in "PEG" are direct.
Another technique is called the peg-word method. Let's say you want to memorize some of the functions of the smooth endoplasmic reticulum: 1) lipid synthesis, 2) detoxification, 3) storage of ions (especially calcium). While the peg word method is especially useful for sequential memorization (i.e. things that need to be memorized in order) it works for nominal data as well. Essentially, what we would do is count off the number of things to be memorized and find simple words that rhyme with those numbers: 1 - sun, 2 - shoe, 3 - tree. Now we will find a way to link these things together - the more outrageous the connection, the easier it will be to remember them! So for one, you could think of a fat man lying out in the sun at the beach - "fat," as you know, is composed of lipids. For two, you can think of using a spray to "detoxify" your smelly shoe. And for three, you can think of a tree that has calcium ions hanging from its branches instead of apples.
The last technique is called the method of loci. In this device, you imagine yourself walking through your house or a familiar building and remember things based on connections to different parts of the house. Lets say we wanted to memorize some of the functions of cholesterol: 1) maintaining the cell membrane at "just the right conditions" (i.e. fluid at low temperatures and not too fluid at high temperatures) and aiding in the production of 2) vitamin D, 3) bile acids, and 4) steroid hormones. When you walk into the kitchen, you see your friend making a cholesterol sandwich between two pieces of bread (like the lipid bilayer) which keeps the sandwich from being too dry or too soggy. You open up the refrigerator and see cholesterol swimming in your milk which, as you know, is usually fortified with vitamin D. In the pantry you see even more cholesterol swimming in vinegar (which is acidic). Finally, in the gym or weight room, you see a bulky cholesterol lifting dumbbells - androgens, as you know, are steroid hormones that aid in muscle growth.
In order to maximize your retention, there are a few study habits that you should try to keep. First, have short bursts of study sessions instead of extremely long ones. Ebbinghaus described a phenomenon that is known as the "primacy" and "recency" effect (or the serial positioning effect) in which we are more likely to remember the beginning and end of a list rather than the middle. Splitting your sessions into smaller ones cuts out the middles and adds many more beginnings and ends. Next, space out your study sessions rather than cramming right before an exam. Ebbinghaus demonstrated that memory was improved under such conditions and dubbed it as the "spacing effect."
However, as a reminder, it is always better to understand what you are learning rather than forcing it into your brain without giving it a network to weave into. Using a combination of these methods in addition to focusing on comprehension will help you succeed in your courses.