We often use miter saws on the ground simply because when we make a cut one end of the board or the other is going to fall down, and falling from a table or a stand makes everything difficult if not dangerous to controll. Many of us carry little blocks just the height of the miter box table to set out to support long boards.
Here is a portable work centre that folds up like a suitcase. When you fold it out, it is a sturdy miter box stand with wings on each side to support the work. Less than a minute to open it up, less than a minute to fold it away. Invented and manufactured in Calgary, it is called Stand Rite.
For a full look at all types of miter saw stands, click here.
For a video clip that goes into detail on making your own extended supports you can check out Miter Saws in the Learning Curve Tab above or simply click here.