Note: I no longer recommend keeping leopard geckos on sand except under specific circumstances. If you feed your geckos in a box separate from their living container, then keeping them on sand is acceptable. Otherwise, geckos should not be kept on sand substrates.
All information in the above care-sheets that did not come from the experiences of the author was obtained from The Herpetoculture of Leopard Geckos by Philippe de Vosjoli, Ron Tremper, and Roger Klingenberg.