Ensure students have a firm grasp of the concepts behind both addition and subtraction before you expect memorization. Test students to see if they understand how addition and subtraction work, the logic behind the operations and the way addition and subtraction complement each other. Students should understand that addition and subtraction are opposite operations.
Help children memorize the easy subtraction facts that subtract zero and one to give them some confidence and success. Subtraction facts that subtract the same number from itself are also very simple. Use these most simple subtraction facts to bolster a student's confidence.
Present fact families to the students to help them connect addition and subtraction more firmly. Fact families group numbers together so the relationship between addition and subtraction is easier to conceptualize. For example, in the fact family 3-5-8, 3 + 5 = 8, 5 + 3 = 8, 8 - 3 = 5 and 8 - 5 = 3. Once students become familiar with all of the fact families, both addition and subtraction will come more easily. Make sure students notice the doubles (4 + 4 = 8) as easy subtraction facts to learn.
Teach students how to count backward to solve some subtraction problems; however, do not encourage students to count backward from a number more than three places. For example, to learn the subtraction fact 9 - 2, teach children to count 2 places back from 9 by saying "eight, seven." Show children that this means that 9 - 2 = 7. Do not use this technique to solve problems with subtrahends larger than three.
Show students the tricks of the nines and eights. When you subtract numbers where the digits of the minuend (first number) add up to the subtrahend (second number), the answer is always nine. For example, 12 - 3 = 9 (1 + 2 = 3). When you subtract numbers where the digits of the minuend add up to the number immediately preceding the subtrahend, the answer is always eight. For example, 15 - 7 = 8 (1 + 5 = 6).
Drill daily after students learn all the facts and tricks for subtraction. Memorization requires daily reinforcement to get the facts firmly seated in the brain. As a student memorizes facts, make separate groups of facts -- those the student knows and those the student does not know. Spend extra time on the unknown facts, but continue reinforcing the facts the student knows also.
Give subtraction drill tests to further test and drill the facts. Include only small groups of numbers for beginning drill tests or include all the subtraction facts for more advanced testing and drilling.