Isopropyl Alcohol cut with water works good also. I just like windex because that acid is what makes the latex in the water based paint break down. Be aware... don't use any kind of heavy solvent on latex paint. IE Acetone, Xylol, Tolulene. these solvents will instantly solidify the latex and turn your brush into a gummy mess. I learned this by expierence
In response to ljcoz:
As I posted before Amonia is corrosive to copper and the brass used in the construction of most brushes is an alloy of copper and tin. The amonia will slowly corrode the brass and make the brush function less efficiently.