As has already been said, let the pigment settle, and then open the bottle a couple hours to let the medium evaporate (or pour the liquid as redzed suggests).
Another cheap and quick option is to add a few drops of good quality white glue (it's an acrilic medium, didn't you know?) to the paint. It dries clear so don't changes the colour, but may make it a little "glossy".
Hope that helps.