Klean-Strip Brush Cleaner
I've used this product for years to salvage paint brushes of all types and it works just as well on natural bristle brushes as it does with synthetics. Likewise, it works just as well with oil-based paints or latex. Just leave any brush soaking in it for a little while and it will soften the paint, making it possible to clean out while the product itself cleans out of the brush with soap and water. This product not only cleans bristles, but conditions them as well. While not the most environmentally safe, it is excellent at what it does; just make sure to use in a well-ventilated area.