I've been working with acrylic paints for one decade, and if you use acrylic paint, you know how easy it is to forget to clean the brush to find it impossible remove the hard-dried paints on brushes. Over the years I tried different products, but if you want to save your time and recover your brush, this is the best product.
Winsor & Newton's "Brush Cleaner & Restorer" comes in three different sizes. The bottle design might look different from the picture below, but as long as it's Winsor & Newton's "Brush Cleaner & Restorer" that's what you need. This product can be bought from different online and offline stores, but here is the link; http://www.dickblick.com/products/winsor-and-newton-brush-cleaner-and-restorer/
Be sure to use this in a glass or ceramic container as this liquid may melt certain plastics, especially those disposable plastic cups for drink.