Thin the paint. Getting the proper paint/thinner ratio in brush painting is just as important as it is in airbrushing. We have had numerous threads about thinning for airbrushing, but indeed thinning for brush painting is a neglected discussion.
I used to use Testors airbrush thinner for thinning MM, but lately I have been unable to find it in hobby shops. So I am now using hardware store thinner. Do NOT just use stuff labeled "paint thinner". It may be any of several substances, some no good. Hold out for either mineral spirits or turpentine. As starting mixes, I use about 2:1 paint/thinner, versus the 1:1 I use in airbrush.
As you use paint, the thinner portion tends to evaporate, so as the bottle gets older, I have to increase the amount of thinner.