The wine stoppers are a fine idea, especially if you are recycling a bottle that can use it.
Some people add marbles to their bottles to "top up" the fluid in bottle and eliminate any air space before capping, that works with any cap. Others use accordion style collapsable bottles.
For bottles holding small amounts of chemical I just displace the air with a shot propane/butane.
My favorite method of storage of prepared chemicals is a "wine box"; once it's empty it gets a good bath, then I make up the chemical and pour it in the bladder, reinsert the valve, expel all the air, then put it back in the box. This allows dispensing without introducing any air into the storage container.
All of these methods help and are worth doing, but none give me the confidence to go much past the manufacturers recommended storage times.
Simply put, the cost of the chemicals is so small compared to the rest the costs of getting a shot that it simply isn't worth messing with.