Sand

 

I've been told you need clay in the sand to make it pack well, but the general rule is,  finer sand with no debri is best.  Unless you bring your own sand you just have to work with what you've got.   On my beach there are lots of sticks and other debri.  If you don't filter them out you can expect clumps to fall out or worse a full collapse. 

    There are lots of sand how to's online.  Here is one I like     http://www.sandscapes.com/how_to/