Frequently when I DM I will give my players consumables (i.e. potions and scrolls) in loot drops in place of gold. My intention is to give them more flexibility in strategy during exploration and combat, especially by providing spells most spellcasters deem too "situational" to be useful (for example, a one-use scroll of Gust of Wind might slow down golbin reinforcements in a hallway, or provide extra force to a sail).

The problem is: they never use the consumable items. The main reason seems to be the players abstain from using the items, waiting instead for the "perfect time" to use them, only to discover that time never comes, and the item is forgotten long after it is found.

What are some methods other game systems or DMs have used to encourage players using these items more frequently? I thought about giving the items "time limits" (i.e. that potion will only last for one in-game day), but then it is hard to explain how it was still good up until the party found it. I am looking less for opinions, and more for empirical evidence of success in encouraging less frugal item use.

To clarify, I am not looking to force a play-style on my players. If they wish to forgo item use, that is their prerogative. I just want it to be for a reason other than waiting for the "perfect moment".

TL;DR

What are some methods other game systems or DMs have used to encourage players to be less frugal with consumable items?

This is a behavior I myself have, and it often boils down to not wanting to use consumable items until my class features -- daily, short rest, or whatever -- have been burned. I hate to use something non-recoverable before something that is recoverable, and even then I feel like it's a waste of the item if the battle ends right after I use it and it didn't have a significant impact. (That is, even if my spell scroll contributed to the battle, if it wasn't an earth-shaking change to the fight, I'm left feeling like maybe I should've just used a cantrip instead.)

In addition, there's often a sense that the action cost is too high, for some items. A potion of healing that recovers less than the damage I took in a single round of enemy attacks doesn't feel like I'm making a good choice. I'm likely to try to push through even with 2 HP left, because spending an action to get myself up to 9 HP feels silly if the enemy is dealing 8 to 12 damage per hit. With or without the potion, I'm just as unconscious either way, and attacking instead might just end things here and now. And after the fight, do I really want to heal with a potion? I'm pretty beat up and we're out of healing spells, so we could find a way to sleep for the night instead...

This tends to get worse over time, as well. The longer you hold an item, the less useful it is, as the opportunity cost of using that item becomes higher. As the player character levels up, the consumable items you got several levels ago become weak compared to your improved innate powers. You're even more likely to say, "Why should I waste an action on this?!"

So how can you deal with this? I can think of a few potential options.

Give out fewer consumable items, but choose stronger ones.

If the issue is feeling that consumables aren't worth spending unless they have a big impact, play into that. Instead of giving the players four potions of healing, give them one potion of greater healing that really can take somebody from near-dead to full HP (adjust which potions I'm talking about to fit your party's level, of course). Instead of handing out a selection of on-level spell scrolls, let them find a scroll for a single spell that's a few levels higher than the players, something they need an ability check to cast. It's a risk, but if it works you turn the tide in a serious way.

If the players' hesitation is indeed based on the "save it for a rainy day" effect, then this could have the unintended effect of making the players even more hesitant to use their sweet item; but on the other hand, the fact that the effect isn't that great can be a big part of it never being "the right time" to use the item.

Put a time limit on the items.

I really like the idea of, say, healing potions that are only good for a few days before they go bad. As you said, it wouldn't make a lot of sense as treasure in a dungeon, but as something a local apothecary whipped up in thanks for the party's actions, it would be a great fit.

It wouldn't be a permanent thing, but you could place an adventure in an area with a pervasive aura of rot or a drained-magic area where non-permanent items start to degrade the longer you stay, so the items have to be used or lost. This could help get the party into the mind-set of using up their stored items, at least.

Remind the players.

Sometimes it's not that you really are thinking, "Oh yes, my feather token could help here -- nah, better not." Sometimes it's just that you forget the item list on your character sheet when you're in the middle of combat. You're deep in "What would a Cleric do here?" mode and don't really consider that second sheet that's off to the side of your spell list. If called on it later, you might come up with an excuse like, "Well, it wasn't the right time" rather than admit that you just forgot you had the dang thing. So sometimes maybe it'd be helpful as DM to just say, "Hey, remember you have an item that would be perfect here."

Provide NPC companions who can use the items.

If the problem is less "saving for a rainy day" and more "action economy and underwhelming effect", an NPC companion who doesn't have other options at hand might be a good way to burn off those items. An apprentice spellcaster who only has level 1 spells, but can use a scroll, wave a token, or feed potions to unconscious party members can be a good way to get use out of those items without forcing the player characters to actively use them.

Let them sell the items.

In some cases there's just nothing you can do. Like your stack of 48 hi-potions in Final Fantasy, sometimes you just have items you will never get around to using, and that's just the way it is. It's not my favorite thing, but at some point, maybe you just need to give the players a way to convert their loads of consumables into something less consumable that might actually make them excited.

Allowing them a way to convert a whole bag full of stuff they don't care for into a single wand of web or something may not be a bad idea; charged items often fix a lot of the problems I discussed, because charges feel a lot more like "use it or lose it" and are now more renewable than, or at least as renewable as, daily character resources. If you do this, I suggest limiting their options, and picking out items to offer that have more than merely combat use, which can help break out of that 'action economy' problem.

Another option is gifting those items to NPCs. Instead of selling them for gold or in exchange for other items at the Fantasy Swap Shop, handing over "weak" items to a local noble or organization might earn them boons and good-will that money might not be able to match. A bag of trinkets that the party doesn't really want could be a huge benefit to an army or constabulary, and might earn them favors, letters of introduction, or anything a bribe could do.

If players perceive scarcity, they are more likely to hold on to items. If they do not encounter many shops or find much gold, items will hold more apparent value. This is basic economics and lots of players will fall into this pattern of miserliness.

Make sure to illustrate the availability/abundance of goods as players move through the world, to mitigate their fear of scarcity. When they are in a market, the vendors can shout "Get your fancy adventuring items here! Best in town!" Have the PCs find plenty of gold when they are questing, in hoards, on bodies, as rewards for jobs.

You can also demonstrate NPCs--either combatants or non-combatants--making use of items to drive home that items are designed to be used.

There are many ways to encourage this behavior

Make consumables abundant

One method you can use is to make make consumables abundant. If they can always get some more healing potions at their local magic shop, and already have 5 from the treasure chest they looted, then they might be willing to use one when another character is making death saving throws. Its a lot easier to use something if you know you can easily get another.

Create a good opportunity for them to use it

You mentioned that they wait for the perfect opportunity to use such items. In that case, creating that opportunity would make such uses more frequent. If a character needs to make a diversion, a burning hands spell scroll would be useful. So put them in a position where a diversion needs to be made.

Make them not super amazing

It is pretty tough so use a once-in-a-lifetime-god-tier magic item, because it is worth a lot of money, and once it is gone, it is gone. It is a lot easier to lose less expensive stuff.

Potential buffs to items?

I know that this is the opposite of what i just said, but this is helpful in some scenarios. A common house-rule is that potions can be used as a bonus action instead of an action, so this helps make it much more practical to use in combat.