The simple answer... more metal in the ground.
When you have unfavorable soil resistivity, the simplest solution is to provide a more extensive grounding electrode arrangement. You must create a larger surface area of soil contact by using more substantial grounding materials, a greater amount of grounding materials, or deeper grounding materials.
Another solution is attempting to change the soil characteristics with enhancement materials and/or moisture. Both of which requires longterm maintenance and constant monitoring, not to mention greater cost and less effectiveness.
Bryan P. Holland, MCP
NEMA - Codes & Standards