To save water and help roots grow, apply a highly organic soil of at least four (4) inches over the sandy soil. Adding a two (2) inch layer of clay into the subsoil will greatly improve water retention. Spreading a "salt free" fertilizer on the soil will encourage strong root growth. If necessary, add lime to raise the pH to a level between 6.5 and 7.
The next step is to till your soil mixture to a minimum depth of six (6) inches. You now have a good, rich seedbed that will help grass send roots deep to capture the nutrients needed for growth. This also reduces water and fertilizer waste due to evaporation and harmful runoff.