other mediums
Future works pretty good, but I prefer to use an acrylic flow agent. Irregardless of the product, the reason these products work is that they contain a detergent which binds to the water base and loosens the bonds between water molecules. This breaks the surface tension and allows it to flow more easily over the figure.
The reason I use a product made especially for acryllic paints is that (IMO mind you) it suspends the pigment more evenly. I also find that it makes the paint more durable, but, as TAB says, some people find this quality in Future as well.