Well, the developing agents are really the only organics we deal with in DIY photochemistry that decays.
I keep mine under glass in small sealed jars, in dim or dark conditions at a near constant temperature, and don't have a lot of problems.
Glycin is famous for short life. That is why the Formulary mixes it up so regularly. I keep mine in the freezer, in glass and it seems to live well for at least a few years that way.
Hydrquinone dry, and metol dry, and phenidone dry and in methanol I treat by just keeping them in glass on the shelf in my mostly dark about 20C all year laundry room.
The pyrogallol, catetcol and ppd and also thoirea (not a developing agent, but an organic I think) I keep in the freezer becasue I use them so infrequently.
The cd-3 and cd-4 I keep at room temperature, although I really should freeze some of the cd-3, because I use it so infreqentlly these days.
I should likely keep my ammonium persulphate reducer in the freezer, because it does die off with age. I am not chemically literate enough to know why though.
Ammonium thiocyanate (some rapid fixers) is a wicked deliquescent, so I mix it to a weight per volume stock solution as soon as I get it.
The hydroxides (if you use them) suck carbon dioxide out of the air, as well as water vapour, to become wet carbonates. So for me they usually stay well sealed, or as saturated about 40% for me w/v stock solutions in glass (dedicated glass - it gradually dissolves somewhat too).
I hope this less than rigourous listing is a start that can be built on more rigourously by thouse with more experience. My DIY knowledge has been built in the last 5 years, but some of my raw chem supplies I have bought up from old darkroom and defunct chem supplier lots and are a lot older than that.
