Glycol is often used. Generally propylene glycol. TEA or DEA are also sometimes used since they double as a buffer and activator, whereas glycol doesn't affect pH once the concentrate is diluted with water for use.
Keep in mind that solubility of the constituents of the developer will vary depending on the medium used. This puts a limit on how concentrated your developer concentrate can be made. Some formulas can be mixed in a very concentrated form (e.g. pyrocat in glycol), while others will have to be more voluminous to allow everything to dissolve.
Some very specific developers like original HC110 (the syrup) used particular 'chemical tricks' that are out of reach of home users; this is also something to keep in mind in case you're setting out to make a HC110 substitute.
As to preserving fixer, are you referring to preserving HYPO (sodium) or RAPID FIXER (ammonium)? Those are two different beasts.
Are you trying to preserve premixed, off-the-shelf "stuff", or is this "stuff" that you are mixing from scratch?
For example, store-bought RAPID FIXER deteriorates on its own, while if you are mixing from scratch what's the point of preservation? To mix a large batch to last a long time?
We are better able to help if we have more information about your goals.
Diethanolamine Sulfite as was used in the old viscous version of HC-110, also the old version of T-max developer and I believe Technidol.
It is now not sold for amateur use and it is I believe hard to find any company that still makes it.
Diethanolamine sulfite | CAS 63149-47-3 REF CSP105749406035 - structural formula, chemical names, physical and chemical properties, references, safety/hazards/toxicity information, supplier lists, and more.
xkeas
"stuff" that I am mixing from scratch--Fix is one at a high concentration.
and yes HC110 (the syrup) comes to mind...
thanks Alen Johnson for the tip.
250gr Hypo [penda-hydrate] to 1lt water is a normal fix and then dilute 1:1 = working solution .
"high concentration" 600gr Hypo [penda-hydrate] to 1.3lt water. then dilute as needed.
FWIW, my understanding is that the sulfur adduct that was used in the manufacture of the old version of HC-110 (and probably T-Max developer) required manufacturing techniques, materials and equipment that are both difficult and expensive to employ, and are most likely not within the range of possible use by people who do not have access to same - they aren't/weren't the sort of thing that one could do at home or in a readily accessible chemistry lab.