We normally assume that memories fade with the passage of time. The more time has elapsed, the more our memories decay. This is only partly true. The opposite is true as well: the more one repeats a memory, the more deeply is it etched into one's mind and the more easily is it accessed. Thus time can either weaken or strengthen memory.
In regards to the levels of memory mentioned earlier this implies that the more transient types of memory can become more enduring ones through repetition. Hence Iconic/Echoic memory can become part of working memory by repeating the sounds or images again and again. It may continue onto short term memory as well, especially if in addition to repetition one makes intellectual sense of it. It may even penetrate long term memory if one finds a way to effectively integrate it with information/experiences already stored in long term memory. The layers of memory are thus dynamic, and through repetition and understanding one can transform even the most fleeting and short lived memories into long term ones.
One factor that determines how deeply a memory will be absorbed is one's level of desire to internalize it. The stronger one's drive to bond with it, the more one will repeat it over and over in an attempt to perpetuate it within one's being. This is visible as a vice in the mental state of 'obsession' where one cannot banish certain thoughts out of one's mind and repeats them consummately. King David referred to the virtuous parallel of this state when he said, "I delight in Your laws, hence I will not forget Your words." He links memory with his level of delight in the subject matter he seeks to remember, knowing that his desire to bond with the teachings will result in their internalization into his long term memory.
The expression 'practice makes perfect' is thus only partially true. For 'practice makes permanent' as well...