Make number lines in class on long sheets of paper. Challenge students to go as far into negative numbers as there is space on the paper as well as an equal amount of positive numbers. Let them decorate and individualize their number lines and take them out to use as a reference when working with negative number problems.
Make the students into a human number line, with each person representing a number. If there are 30 students in the class, they can be -75 to +75 in increments of 5. Write an equation on the board involving negative numbers and ask the two students who are the numbers in the equation to step forward. Then, the person who is the correct answer must sit down. For example, write -25 + 10. The person who represents -15 sits down. Continue until everybody has had a turn to step forward or sit down. Most sixth graders are in their first year with no recess period. A human number line lets them move around the room while still learning.
Make more complex problems for your students to solve when learning to write and solve equations using x and y. Ask students to create a number line that contains all the numbers used in a problem, then circle the numbers that solve the problem. This bridges the gap between the elementary school "story problem" with pictures and the more abstract middle school algebra exercise.
There are several educational games available on the Internet for free that let students work with number lines. Let your students play these games in math class or as homework; they will help solidify the concept of negative numbers and moving from negative to positive.
Make fraction number lines that go from negative to positive. Use one large number line at the front of the class or have students make their own. Then, do problems as a class that add, subtract, multiply and divide fractions. Challenge students to write and solve their own fraction equations using the number line.