It is a common belief that memory retrieval is a simple process. Information is stored in working memory, transferred to long term memory, and can then be retrieved as needed. However, the reality of memory retrieval is more complex than this simple, certain path. Just because a memory has been fully encoded is no guarantee that it can be retrieved and applied at will. There are many factors that affect the retrieval process that can make us either more or less likely to remember information when we need it.
One process that can improve the likelihood of remembering previously learned knowledge are retrieval cues. Retrieval cues are any stimulus or words that help us remember stored memories (Goldstein, 2011). These cues can be just about any sort of stimulus, from familiar sounds, to sights, to smells. These cues can be surprisingly powerful and can help us remember events we may not have thought of for years, such as returning to a childhood home and recalling many events from time spent there. To demonstrate the effectiveness of retrieval cues, a study conducted by Tulving and Pearlstone in 1996 (Goldstein, 2011) compared free recall (where a participant is simply asked to remember a set of words) and cued recall (where they are provided retrieval cues before being asked to recall a set of words.) The results showed the free recall group could only remember 40% of the words, whereas the cued recall group remembered 75% of the words.
It’s probably not particularly surprising that memory can be improved with clever reminders. However, the conditions in which a memory was encoded also can have a profound impact on when that memory is recalled. When a memory is encoded it is more likely to be recalled in conditions that are similar to the ones in which it was initially learned. This is known as matching conditions. While there are likely many more principles phenomena can be applied to, there are three that are the most well known and studied. First is encoding specificity, which applies to the external environment at the time a memory was encoded, such as smells, sounds, physical characteristics of a room. Second is state-dependent learning, which applies to the individuals internal state or mood at the time, such as their mood or state of awareness. Last is transfer-appropriate processing, which applies to the type of task that was conducted during encoding, such as the sort of reasoning that was being applied. When these traits match up to the ones we were under when we encoded the memory, we are much more likely to remember it later.
This can have significant repercussions in our lives in any situation where it is important to remember large amounts of information (most obviously any study session should consider these facts. When studying, trying to learn the information that is being tested isn’t the only thing that should be considered, but also what characteristics the study environment will have. Consider turning off any music or other distractions, not just because they can be distracting, but also because it is unlikely the test environment will be silent as well. Tailoring your environment to one that matches the desired one could take some effort, but it can make a large difference in recalling needed information.

Goldstein, B. E. (2011). Cognitive Psychology: Connecting Mind Research and Everyday Experience (3rd ed.). Boston, MA: Cengage Learning.