Get some friends together--friends who are actually interested in studying, that is--and have everyone bring over their flash cards. Pass them around and quiz each other. If anyone is unclear on a concept, take turns explaining them to each other. Better yet, turn your study session into a game like Trivial Pursuit.
The most effective way to retain information is to "tie" it to existing information that's already lodged in your mind.
You don't have to memorize them, but whenever there is an important concept in a particular field, there is usually a special term to refer to it. Learn these terms, and be able to use them easily, and you will have gone a long way towards mastering the subject itself. (Besides, teachers frequently draw from these lists as a quick and easy way to make up test questions!)
Traditionally, this is done with index cards, but you can also download computer programs that cut down on space and the cost of index cards. You can also just use a regular piece of paper folded (vertically) in half. Put the questions on the side you can see when the paper is folded; unfold it to see the answers inside. Keep quizzing yourself until you get all the answers right reliably. Remember: "Repetition is the mother of skill.
Don't just read the material from beginning to end, stopping to memorize each new fact as you come to it. New information is acquired much more easily when you can relate it to material that you already know.
When you're in class, emphasize recording over understanding or neatness when you take notes. That doesn't mean you shouldn't try to understand or organize your notes at all; just don't waste time doing something in class that you can figure out or neaten up at home. Consider your in-class notes a "rough draft" of sorts. Rewrite your notes as soon after the class as possible, while the material is fresh in your mind so that you can fill in any gaps completely from memory. The process of rewriting your notes is a more active approach to studying--it engages your mind in a way that just reading the notes doesn't.
Have your snacks prepared when you begin a study session--don't wait till you get hungry and go rummaging for food. Avoid any snacks or drinks that will give you a rush of energy, because with every rush comes a crash in which all the information you studied is lost to an intense desire to sleep. Focus on "slow release" carbohydrates, which not only give you a steady stream of energy, but they also boost serotonin, a brain chemical that makes you feel good.
You should feel comfortable, but not so comfortable that you risk falling asleep--a bed isn't a very good study spot when you're tired! The place where you study should be relatively quiet (traffic outside your window and quiet library conversations are fine, but interrupting siblings and music blasting in the next room are not).
It takes time for your brain to form new long-term memories, and you can't just keep studying flat out. Take 5-10 minute breaks minimum and do something physically active to get your blood flowing and make you more alert. Do a fewjumping jacks, run around your house, play with the dog, whatever it takes. Do just enough to get yourself pumped, but not worn out.
Make a weekly schedule and devote a certain amount of time per day to studying. This will also improve your grades. That amount will vary depending on whether you're in high school or college, and also varies by field of study.
Logical arguments will not give you motivation to study. Thinking that if I study hard and get into a good university and get a good job etc, will not interest you. Love what you do. Try to find the beauty of every subject, and most importantly try to link it with the events of your life. This linking may be conscious (ie. performing chemical reactions, physical experiments or manual mathematics calculations is order to prove a formula) or unconscious (eg. You go to the park and look at the leaves. Then you think to yourself, Hmm, let me review the parts of the leaf we learnt in bio class last week). Even though this might not sound the most ideal method for theoretical subjects such as English, use your creativity to make stuff up. For example try to write a story with all subjects starting with S, all objects starting with O, and no verbs containing V.