I was searching for product recs online and I came across this from a blog I like (15 Minute Beauty)...apparently people use Milk of Magnesia as a way of keeping their face matte all day long.
I don't know that I'll be trying it, but apparently it is a "thing."
I haven't seen it discussed around here...only the Monistat gel used as a primer.
http://www.15minutebeauty.com/2010/05/milk-of-magnesia-secret-to-matte-face.html
http://www.makeupalley.com/product/showreview.asp?ItemID=10870
Re: Milk of Magnesia to keep you matte...
I've been using it for the past couple of weeks, and I love it. I've also noticed a difference in my skin altogether, it's gotten clearer since I started using this. I got the generic bottle of plain milk of magnesia(no flavors), for less than $5, which is an awesome price for a primer.
I shake it up, pour a little into the cap and put a light layer over my face with a foundation brush. If you use too much you'll see the white powder as it dries, you don't need much at all! I've even used a primer over the MOM, and I didn't notice a whole lot of difference appearance-wise.
I highly recommend checking this out if you have oily skin, especially with summer coming up.