Table with 45 degree corner groove
A more typical way to strengthen a miter joint is to use either splines or biscuits.  Splines can be oriented in one of two ways.  It's probably easier to show pictures than to explain in words:

a) Spline running the entire miter.  With this method you can also make the grooves stopped so the spline would not be visible at the edges.  If you are working with MDF you can cut the spline so the grain is running the length of the spline, as opposed to the way it is shown in the example. 



b) Reinforcing with biscuits.  This is similar to (a) except that it requires that you have the biscuit joiner to cut the pockets for the manufactured biscuits.  This is definitely the easiest and fastest of these three options. 



c) Multiple splines cut through the miter.  The third method can be made into a decorative element if the splines are made from a contrasting color of wood.