Developer is a reducing agent, which means it is susceptible to oxidation and therefore should not be exposed to air.
Good containers for storing developer include:
Bottles need not be dark-coloured to exclude light; the chemicals aren't particularly light-sensitive and a cupboard is decent protection.
Bad means to store developer are:
While accordion bottles allow you to push all the air out initially, they are bad on two fronts: they're LDPE and therefore oxygen-permeable, and the corrugations make them practically impossible to keep clean.
Always mark your bottles and caps with a permanent marker to ensure that:
In particular, very tiny quantities of fixer will destroy developer.
Reactions rates are strongly related to temperature, therefore developers keep best when cold and very poorly when hot.
A cool cupboard under 25C is sufficient for B&W developers but refrigeration to under 4C is recommended for the keeping of (expensive) colour developers. C41 developer once diluted has a rated life of only two to four weeks but will keep perfectly for at least four months in an air-free (butane-filled) PETE bottle in a fridge.
Do not freeze developers, it can destroy them.
Most B&W developer concentrates (LC-29, HC-110, Rodinal) keep well without any special attention. Rodinal in particular will last decades and works fine even after some of it has crystallised.
Powdered developer in its original sealed foil bag will keep for a decade or more.
Fixer has no special storage requirements; the concentrate will keep for years and the working solution for months in a cupboard.
Bleaches used in colour and reversal processing actually benefit from oxygenation, therefore require no special storage.
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