According to Valerie Gennari Cooksley author of Aromatherapy...A Lifetime Guide to Healing With Essential Oils, says that the best way for safekeeping your essential oils are
(1) The best way to store your essential oils (EO's)in an amber, cobalt blue or other colored glass bottle to protect them from the sunlight. You also want to keep them out of direct sunlight and away from any heat source.
(2) Keep your EO's well labeled.
(3) Do not use rubber stoppers or rubber eye-dropper type lids to store your EO's over long term. Over time the rubber will softened and destroyed by the EO's gassing off in the bottle. Always keep screw tops on tightly, since the oils will evaporate quickly.
(4) For optimum shelf life it is best to re-bottle your oils if they are half full since the air inside the vacant space encourages oxidation, which means a shorter shelf life.
(5) When a essential oil is mixed with a carrier oil, the shelf life of that preparation will equal of that carrier oil. So whatever oil you select will determine the shelf life of the aromatherapy blend, not the essential oil.