Some believe that flavors and nicotine strength does not diminish in flavor or strength but my theory is:
If I purchase already mixed eJuice, I split the order into smaller bottles rather than buying the larger bottle even if it cost a little more. This way I draw from the one open bottle and the rest stay sealed until ready for use. Also, don't leave the eye dropper in the bottle, remove it after use and seal it back with a cap. I do the same with all my liquids. I buy small bottles, preferably in glass.
When I mix my own recipes, I mix small quantities, so I am mixing all the time and the mixed juice doesn't sit around losing flavor or strength.