Natural soaps contain all their naturally occurring glycerine, which is a humectant or, to put it simply - it is a substance that draws moisture in.  This is a great thing for your skin - but it does mean that your natural soap can, and will, get "goopy" if you leave it in the shower, or on the bottom of the sink.  

Make it last longer, slice a piece off and just use a little bit at a time - wrap up the remainder of the bar and keep it somewhere dry.  Use greaseproof paper or a paper bag - this allows the soap to breathe. Air circulation is important for the longevity of your soap.