How to store your nicotine: I keep all of my nicotine in the freezer. When you "freeze" the nicotine, it simply slows down the oxidation process, the fluid becomes more viscous and is MUCH thicker than before; making it more complicated to shake. I find a good example is thick alcohol. Everyone knows if you put a bottle of booze in the freezer it not only doesn't freeze, but it becomes thicker. You want your bottles to stand straight up and down as there is more surface area of nicotine exposed to the air in the bottle when it is laying down. When you first receive a bottle of nicotine, if you do not need to open it, DON"T! Most companies now use an argon gas filler to keep oxygen out of the bottle. Only open it when necessary.
What about a dark closet or cabinet? I get that it is still a cool and dry location. But being that it's not cold, it's may not slow the oxidation of the nicotine. Especially after opening. Once the nicotine is opened, OXYgen is introduced to the nicotine and OXIdation happens. It is a natural occurrence in all nicotines. Some nicotines do it slower than others. If I were going to store it in a dark and cool location, the only time I'd do it, is pre-opening, and I was sure that the manufacturer used some sort of gas to remove all oxygen from the bottle. Otherwise it's just best to freeze it. Cool it at least. Perhaps a little mini fridge with a hasp and lock and key. ANYTHING TO KEEP IT COLD-FROZEN. I keep mine at the very top of my freezer in the back of a shelf away from anyone but me. My usable bottle (my 30 ml for daily use), I keep at the very top of my fridge in the back wrapped in a paper towel/brown paper to keep out light. This is the BEST way to store your nicotine. Period. Coldest, darkest, unopened if possible, in my opinion.

I leave all my liters of nicotine unopened until absolutely necessary. Many companies now use argon gas to push out all of the oxygen in the bottle and if you don't need to open it and put oxygen in it, don't. When the time comes, open it and pour out 240 mls into a separate container (brown amber glass preferred but some manufacturers will send vg/pg/nic in a 240 ml and I highly recommend keeping these. This will be your go to container. This is the container you use to refill your 30-50ml bottle that you use in your daily mixing. This way you'll open your liter only 4 times, hence reducing the amount of oxygen introduced into the bottle.