Flow improvers, sometimes referred to as "thinners", affect acrylic paint in exactly how 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 their 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 experiment with until it works the way you want.