Given that the fundamental goal of education is to make changes in the learner's long term memory, Bjork et al. have shown that learning conditions that introduce difficulties for the learners are potent in enhancing long-term retention and transfer. Humans do not simply "store" information in long term memory but rather, we relate new information to what is already known. Our long term memory is not a playback device. It is primarily semantic in nature. "Desirable difficulties" have been shown to be effective in making changes in the long term memory. This includes: spacing rather than massing study sessions, interleaving rather than blocking practice on separate topics or tasks; varying how instructional materials are presented or illustrated; reducing feedback; and using tests rather than presentations as learning events. See Bjork's Seven Study Tips and his slide presentation.
Bjork, R. and Linn, M. (ND). Introducing Desirable Difficulties for Educational Applications in Science (IDDEAS). Retrieved on December 3, 2009 from http://iddeas.psych.ucla.edu/IDDEASproposal.pdf.