Although students learn the basics of subtraction in first grade, they must continue to build on that knowledge in second grade and beyond. Most students must learn addition and subtraction facts for numbers 1 through 20 in first grade. Secondgrade students must learn how to regroup numbers in order to subtract multidigit numbers from one another accurately. Fortunately, teachers can use methods other than drills and worksheets to help students understand. Base 10 blocks and other manipulables allow students to visualize math concepts in a concrete way.
Things You'll Need
 Base 10 blocks or other manipulables
Review

Talk to the firstgrade teachers in your school about the methods they used to teach subtraction. Using similar methods in the beginning will be less confusing for your students. If you don't like the methods that the firstgrade teachers used, start by doing things their way and slowly introduce your own methods throughout the year.

Give the students a short pretest subtracting numbers less than 20 to see how well they learned subtraction last year and retained it over the summer.

Devote at least two days to reviewing subtraction of numbers less than 20. Use the methods that are most familiar to your students  fact families, number lines or other strategies their firstgrade teachers used. If the students did poorly on the pretest and you want to try out your own teaching method, introduce it sooner rather than later; part of the problem may have been that the students didn't understand the method they used in first grade.
Teaching

Give your students a unit block from the base 10 blocks. Tell them that this is a "one." Give them nine more "ones" and have them practice subtraction problems using the objects (for instance, have them practice 10  3 by taking away three blocks and counting how many are left).

Have your students line up their blocks into a 10block line. Give them a 10block rod from the base 10 blocks and show the students that they're the same. Have the students practice a few problems subtracting numbers from 20.

Tell your students to put away five of the unit blocks. Explain that they'll be subtracting numbers from 15. Have them subtract four from 15 to doublecheck their mastery of the basic idea.

Ask the students how they could subtract seven from 15 using these number blocks. Once the students say they can't, have them put away the 10rod and give them 10 unit blocks. Explain that it's still 15, but shown in a different way. Have them subtract seven from these 15 unit blocks.

Give your students more 10rods once they're comfortable with this basic concept, allowing them to use numbers bigger than 20 in their subtraction problems. Practice as a whole class at first before moving on to independent work.

Put pictures of base 10 blocks on their first few subtraction quizzes so they can visualize the process more easily. The first link in "Resources" shows some ways to transition students from base 10 blocks to regular problems on tests and quizzes.

Introduce 100cubes in the same way that you introduced 10rods if your state requires that secondgraders learn to add and subtract tripledigit numbers. Wait until they've mastered doubledigit numbers before doing this.
References
 Photo Credit sugar cubes image by poGosha from Fotolia.com