I use Testors acrylics (they are getting harder to find, but Micheals still usually has them). If I want a heavy dirty wash, I will dip the brush in water first then dip it quickly in the paint. Spread the thinned paint up and wipe it off with a rag within a few seconds. The longer you let it set, the less will wipe off.
For a lighter wash, for detailing, I will dip the brush in the paint first, and then dip it in the water. The longer you leave it in the water, the lighter your wash will be, and the thinner the coat will be. However, the thinner the coat, the faster it will dry too, so sometimes you need to work quick.
Finally, don't always use black as your wash color. Sometimes it's best, but for a more sublte wash, go with a color that is just a few shades darker than what you are washing.
I've found these methods to work okay for highlighting too, but I prefer to drybrush for that.