I would do it the way Scott suggests, but if you need a shape that you can resize without squishing or stretching, there is something else you could try in addition to this
Create one half of the design as per Scott's suggestion
Add a Transform reflect effect from the Appearance Panel
If you want to rescale the object, select the end anchors, and move them with the arrow key on your keyboard.
Because the Transform is a live effect, it will extend automatically to the other side, and because you are moving the anchors rather than stretching there won't be any distortion.
An example