Memories, like muscles, must be used to keep in shape. If you study something the night before a test, you may store it in short term memory and pass the test, but the odds are that you will not remember it a few days later. There are several ways to make sure that you retain information in your long term memory and are able to access it effectively.
First, you remember more of what you write than of what you hear or read. Therefore, if you want to remember things for a long time, you should write out your own study guide or summary in your own words.
Next, to recall information, you should keep using it. So, for example, if you want to remember foreign language vocabulary, you need to spend an hour or two a week reading or watching films in that language to keep memories active. the same goes with practising math or writing skills, or remembering history (perhaps watch TV shows about the period).