Steve Sobek demonstrates an awesome math trick that will enable you to easily square whole numbers up to 99. After four examples he proves the validity of the procedure.
The same procedure can be applied to numbers larger than 99, but I would need to remove the word, "easily" from the title unless the number is close to a power of 10. For example, squaring 998 would still be easy by taking 1000 x 996 + 4.
These types of problems can be practiced with online flashcard drills at www.AmysFlashcards.com