One fun way to get students to start thinking about subtraction is to have them complete interactive activities with coins, fruits or other touchable objects. For example, one lesson can have each student hold on to 10 apples. He can then be asked to take away two apples from his collection and report the number of apples remaining. This can be accompanied by documentation so that each student gets in the habit of writing down the outcomes of his activities.
Flashcards can get students to start using subtraction through the format of a game. For this lesson, purchase a deck of subtraction cards that have simple subtraction problems on them or create a deck. This can easily be done with a set of note cards. Write a subtraction problem (such as 10 - 2 = ?) on one side of the card and the answer (8) on the other side. The students can use the cards in pairs. One person can show the question side to the other student and check the answer with that on the back of the card. The student who gets the most questions right can receive a prize.
Once students have a grasp on the idea of subtraction and have had enough time to play with the concept, you can supplement their lessons with speed tests or exercise sheets. These will help students memorize the answers to certain simple subtraction problems instead of relying on counting out answers on their fingers or on paper. This lesson can be implemented daily or weekly as a challenge to the students. Simply list multiple subtraction problems on a sheet of paper and have students complete as many questions as they can on the sheet in the allotted time --- a few minutes.
One of the best ways to teach the actual lessons of subtraction to a first-grader is to demonstrate on the board. Start the lesson with diagrams of a pie divided into four pieces. Familiarize the students with the dash that represents the minus sign by writing it next. Then draw two of the four slices of the pie followed by an equals sign. By using this kind of visual, students will be able to see what the concept of subtraction entails. Ask the students of the class how many pieces will be left. This can be accompanied by an illustration of the remaining pieces. Feel free to continue this exercise with different numbers of slices of pie.