Maximizing the storage life of mixed chemicals isn't all that difficult. Start out with the water from your tap. Chances are that your faucet in the kitchen has a aerator on it to make the water "taste better". At the water plant it was already aerated once because it tends to kill bacteria. Then various other chemicals are added to make it safe and "pure".
Put your water in a big pot on the stove and bring it to a slow rolling boil for about 15 minutes. This will drive off most all the disolved gases, like oxygen, that will spoil your developer. Let it cool. When you mix the developer stir until it's all desolved, but don't whip it to a froth. Divide the solution into smaller bottles filled to the very brim before capping them. Ordinary plastic soda pop bottles are perfect. If you drink the soda the bottle is free. At the worst, you can buy two liter bottles of store brand soda for 69 cents or so and pour it down the drain. Soda bottles have to be able to withstand the presure that the disolved carbon dioxide makes when the bottle gets shaken. That plastic will keep out oxygen too. If you're mixing and using smaller quantities of chemicals start out by getting smaller bottles of soda. Store the bottles in a dark room, closet or such so they don't get a lot of light. Everytime you mix chemicals and store them this way you end up with three brim full bottles and one that's maybe 75% full, which you use first.
