Sometimes the final arithmetic skill for students is the practice of "borrowing," which is necessary in many multi-digit subtraction problems. To help your students get used to borrowing digits, give them a few problems with unusually large numbers or several problems subtracting values with three to four digits each. In addition, make sure that your students are equally comfortable borrowing over zeros. For example, when solving for 2000-1997, students will need to borrow over three zeros. If you organize each drill to focus on a certain skill, such as borrowing with integers and borrowing over zeros, you'll have an easier time identifying individual students' abilities and difficulties.
For a slightly less traditional drill, give students a series of short story problems. If you hand out similar story problems regularly and you establish them as a brief, fast-paced activity, you won't just teach your students subtraction skills; you'll also help them to rapidly interpret the mathematics behind a story problem format. For example, you might give stories that use large groups of people, goods or sums of money. By giving values with three or more digits, you'll help third-graders drill written column subtraction.
To make mental math more intuitive, give your students subtraction problems that they can break down using regular patterns. For example, instead of a traditional list of mathematical expressions, your drill might pose a series of written questions, such as "How is subtracting 200 from 900 like subtracting 2 from 9?" or How could you check your work if you wrote that 70 minus 40 equals 30?" When grading, be open to students' varied methods of expressing their logic.
To help your students practice their mental subtraction abilities, have them spend five minutes answering simple, rapid subtraction questions. To play "Round the World," have two students stand side-by-side and ask them a simple subtraction question; see who answers first. The student who can first answer correctly then moves to stand beside the next student and repeat the drill. Whichever student can travel farthest around the classroom, or "Round the World," wins.