Heat Storage

Old indian concept:  Dirt is a great insulator and stone can store a heap of heat.  
1)  Dig a large hole in an area that won't kill the trees via heat leaching or get drowned out by underground water supplies:  6' deep (that's about 2' below the maximum winter frost line around here) x 10' long x 4' wide (that should be more than sufficient).
2)  Lay down a layer of crushed stone in the bottom about 1' thick.
3)  lay about 100' of pipe or tubing or hose or whatever you feel would make a good heat exchanger line for your hot water to flow in.
4)  make sure the ends come up out of the ground.
5)  Lay down another 1' thick layer of crushed stone on top of the piping.
6)  Fill the hole back in with soil.
7)  Hook up to your solar heater.

It will take about as long for the storage unit to reach temperature as it will store, so be patient.  Possibly as much as a month.