Flow improvers, sometimes referred to as "thinners", affect acrylic paint in exactly the manner in which their name implies -- they enhance fluidity and "flow." Unlike water, however, flow improvers don't dilute the pigments within the paint but instead work to maintain color strength and vibrancy by suspending the pigment in solution rather than allowing it to sink, as it will in water.
Flow Improver:
Improves the flow of paint
Does not diminish the strength of color
Improves leveling properties
Makes covering large areas easier
Mix flow improver into acrylic paints to improve their flow without diminishing the strength of its color. Improves leveling properties and makes covering large areas with color easier. Will not cause separation of binder and pigment.

So, just add a drop or two into your paint when you need your palette to stay wetter longer. It's a great product to play around with and experiment until you get it to work the way you want.