Use flashcards.

One of the best tips we can give you is to make and use flashcards. Make cards that are small enough to easily carry with you, and write the English on one side and Spanish on the other. Be sure to ALWAYS have some cards with you. This way, you can capture "wasted" time (standing in line, riding the bus, waiting for class to start, etc.) and turn it into productive study time. Even if you only have a couple of minutes, you can use it to study a few flashcards.

http://www.studyspanish.com/topten_tips.htm#five

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