Algebra Readiness: How can you help students understand the concept of variables when they are just beginning to learn algebra?

In my experience, this can be done early on with students learning basic addition and subtraction facts. One of the things that I often do with my math intervention students is discuss the importance of an equal sign. I will do this by incorporating a scale and have them put manipulatives (bears, base ten blocks, counters, etc) in both sides to show that equal amounts are on both sides and that the scale balances. Then we talk about missing addend problems (3 + ____ = 7) and we build it, with 7 on one side, 3 on the other and they add the manipulatives until it balances. Once we do this, I will put an object (or a letter) in for the blank (3 + a = 7) and we discuss that a=4. This is a great way to set up students for success later on!

What ideas do you use to help students?