Always store essential oils in dark amber, cobalt blue, or violet glass bottles.  The dark glass helps to protect the oil from ultraviolet rays that can significantly decrease the storage life of the oil.  Keep the bottles in a cool, dark place away from heater vents and registers.  Avoid temperature fluctuations as much as possible.

The oils, as used, should be transferred to smaller bottles to reduce the amount of oxygen to which they are exposed.   Minimize the time the bottles are open and keep them snugly closed.

The lifetime of most oils is between 6 months to several years.  Citrus oils are the most susceptible to early breakdown.  As they get older, you may notice clouding or a 'sweetening' of the fragrance.  Once they reach this point, their use for aromatherapy purposes is significantly reduced.   However, you can still use them for freshening of a room, ...  Oils such as Sandalwood, Frankincense, Patchouli, and Myrrh have a much longer 'shelf' life.  Many people have had these oils for years, with little reduction in their properties.  Of course, with evaporation, they will tend to thicken and may darken.

When using the essential oils, avoid contaminating the oil; don't touch to top of the orifice reducer or inside of the cap.   If using pipettes to transfer the oils, use a separate pipette for each oil.   Some use small individual plastic bags, labeled for each oil, to store their pipettes.  This may work for a short time (several days perhaps), but the oil in the pipette is exposed to oxygen in the bag and will deteriorate quickly.  By using this pipette again, you risk contaminating your entire bottle.