The tip others have mentioned of adding a drop of dishwashing liquid to the wash can help with that, too, I've found.

Drifting off topic a little, but with a drop of dishwashing liquid, a drop of water, and some ground pastel chalks, and I can make a sludge for weathering.  It looks like mud stuck or caked on the sprockets and bogeys of a tank, for example.  I have used an old paintbrush to apply it, as well as coffee stirring sticks and toothpicks.