To solve the problem of having to mix up too much developer at one time I suggest a solution involving two developers. For slow and medium speed films you can make up PC TEA. There is another thread on PC TEA active now. It is made up of phenidone, ascorbic acid (vitamin C) and triethanolamine (TEA). PC TEA is inexpensive and lasts a long time. It may be too grainy for some people with fast films. For the fast films you can make up D-23. D-23 is made of metol and sodium sulfite. It's easier to make than PC TEA or D-76 and a set of measuring spoons will give adequate accuracy when "weighing out" the two chemicals. D-23 is soft working and will give results which are similar to what you get with D-76. Buying powder film developers in 1 quart or 1 liter size is expensive. Buying in larger sizes may leave you with more then you can use right away.
If you don't like mixing things yourself from scratch then HC-110 is a good choice. It lasts a long time if you mix up the working solution directly from the concentrate. Freestyle sells its own versionof HC-110 under the Kentmere name and that ay cost less than HC-110.
The general advice is don't do it.
However, those who have done it report that they haven't encountered any problems.
I haven't done it - I store commercial developer in 1 liter bottles or mix small batches as needed from scratch chemicals. I don't know the powder will keep any better than the stock solution - I have had D-76 last a year in Nalgene bottles.
Here's a plan. Mind you, I haven't actually tried this, so it is theoretical.
First, mix the powder as best you can by whatever method you think is best.
Next, pour it into a big pile.
Next, find yourself a small spoon or scoop.
Next, fill the spoon.
Next, pour the spoonfull into a another location away from the pile. Let's call it location 1.
Next, fill the spoon again.
Next, pour it into location two.
Keep doing this until you have several new piles. Let's say four for sake of discussion.
Next, take another spoonfull and pour it into the pile at location 1. Now you have two scoops in location 1.
Now, put another scoop into the pile at location 2.
I think you are getting the idea.
This will divide the original pile up into several new smaller piles of developer. It should produce a pretty uniform composition between the piles, even if the original pile was somewhat inhomogeneous.
Don't do it. It's not a good idea. Getting an even distribution of all the components that make up the developer may or may not be a problem, but that doesn't matter. As sold, the developer is packaged in an airtight envelope with an dry and inert gas. Once you open the package, the contents are exposed to oxygen and moisture which will start to degrade the developer in short order. It will last longer if you mix the whole thing up as directed, and seal it up in 1L or smaller bottles that are full to the brim. Do that and the developer will last at least 6 months and likely more than that. Partially full bottles don't last as long, but you should be able to finish them up before they start to go off in a couple of months.