In an effort to keep my acrylic paintings fresh, I approach them with what I call "Watercolor Mind." Watercolors can easily be overdone and become muddy, so the artist must know when to quit working on a passage. My college watercolor professor Michael Frary used to say that a passage in watercolor could only be painted over twice without risk of becoming muddy.
With acrylics, it's a different story; passages can be painted over several times without harming the painting too much. For the perfectionist, this creates the problem of not knowing when to quit. Even though acrylics allow more chances to paint over an area than watercolors, freshness can be lost. Repeated layers of paint can cause the surface of the canvas to become slick and difficult to paint on. Despite the extra leeway, the best way to approach a forgiving medium like acrylics (or oils) is still to make freshness a priority.
However, if you do end up with slick areas on an acrylic painting, there is a remedy. These passages can be resurfaced using acrylic modelling paste and a coarse brush. Allow to dry completely before painting over it. Note: this resurfacing method cannot be used with oils.