MoM is actually sold as an antacid/laxative, in the pharmacy section of most drugstores. It can be used, however, as a fantastic oil-absorbing mask, or even a mattifier under makeup (diluted with water). All you need is any unflavored version. Apply to the face with fingers, cotton, or a brush in a thick layer. Let dry for about 10-15 min, then rinse off! Your face will be smooth and oil-free.
And cheap too! Paula's Choice has an Oil-Absorbing mask that does the exact same thing. Except it costs a lot more. Strangely, Paula's website mentions that their mask has the same ingredients as Milk of Magnesia (only with an easier to apply base so it isn't so liquidy). I ordered the mask, but I don't see any reason to do that. Just buy a bottle of Milk of Magnesia, and you have two good things in one bottle!