Question

What would be the best technique to separate an insoluble solid from an liquid, like sand from water

Answer 1

Sand can be separated from a mixture of sand and water using filtration. That's because sand does not dissolve in water.

Answer 2

The best technique would be to use filtration to separate sand from water. Pass the sand water through a filter paper. Since sand is insoluble in water it will get collected as residue in the filter paper. The water will pass through the filter and get collected as filtrate.  

Other technique could be decantation, as sand is heavier and insoluble in water, sand particles will settle down in the sand water mixture taken in container. The water can be collected from the top after the sand has settled at the bottom.