Accumulation or Collection, I personally feel, is one of the "hidden"
Aesthetics of Play that can drive a fun experience for a game built around it but is rarely talked about as an Aesthetic of Play in game design theory. So it's worth thinking about the fact that finding and
keeping rare items can be very satisfying to some players, and that if they have to use that rare item then the player almost feels like they are sabotaging their own collection. In a lot of cases you will not
want to discourage the player from hoarding their rare consumables!
Like all other Aesthetics of Play, though, it can be pretty safely ignored where the game is instead focused around a few
other Aesthetics of Play such as Challenge (improvising ways to overcome arbitrary obstacles) or Expression (customizing the experience based on the player's own creativity and values). You don't need to offer filling out a collection as a core Dynamic - and if you choose not to, then (getting back to the main idea of this thread) it's worth thinking about how to
encourage the player to use their consumables (rather than discourage their hoarding - always think in terms of motivations not penalties).
One of the best ways to encourage players to use a resource is to place a "
Shot Clock" on the resource - use it before X, or else it will either be wasted or won't be an efficient use of the resource. Some games do have items "spoil" after a certain period of time, but I find that creates a poor experience because it requires the player to keep checking time remaining, comparing it to other items, and generally spending a lot of unnecessary time in the menus. Rather, very clear and obvious shot clocks such as "lasts until the end of this battle" (for items created inside combat) or "lasts until the end of this dungeon" (for any linear games) make great choices. The player may hoard them for a few turns or a few battles but it is always with the strategic intention they will use it before the end of the battle or the dungeon. Similarly, imagine a game where you can collect power-ups that give you a small passive bonus while held but then can be consumed for some incredible (and flashy) effect. In order to ensure the player uses these consume effects, a designer might introduce a mechanic where collecting another power-up will completely replace the held one. That encourages the player to consume a power-up any time it's useful, as they may not find a better time to do so before finding the next power-up.
Long-term Shot Clocks are rarer success cases, but there are ways they can be effective as well. In a game where you have, for example, a limited number of in-game days to accomplish a goal, it becomes more justifiable in some players' minds to use their resources rather than to try and go without. For example, in
Persona 4, if you had unlimited days to complete a dungeon, you'd hoard your MP restoring items (which are rare and are the only way to refill MP without losing a day by returning home and resting). But because you only have a limited number of days to work with, sometimes you will want to push a little further to complete a part of the dungeon, and it will feel like a good investment to consume some of those rare MP-restore items to make that final push.
Recettear does something similar with payment deadlines you have to reach along the way (every couple of in-game weeks). Instead of hoarding every item you find, you have to sell most of them off to earn money and make the deadlines - missing one is the only way to get a Game Over!
Another good way to encourage players to use a resource is to
combine Plentitude with Hard Caps. Plentitude meaning the resource is very easy to find or refill; Hard Caps meaning you can't hold more than a certain amount of them. This is often done in modern games - to great success - with MP. Most games make MP very easy to recover, sometimes recovering 100% of Max MP after each and every battle. However, since you can't hoard MP between battles to start the boss with 600% of your Max MP, there's no reason to hold back with your Skills unless you think you'll need that MP on a later turn within the same combat. Encouraging the player to sling skills freely is good, because using skills is fun! The same concept applies to some extent with Hard Caps on plentiful consumable items - for example, a game where you can buy Potions for a few gold at shops around the world but you can't carry more than 5 Potions at once. This encourages the player to use those Potions whenever they feel they need them - more will always be there for him when he gets back to town, and it's not like he has permanently made his collection any worse by using those Potions. Just be very cautious about introducing a limit on your entire inventory (e.g. your backpack can only hold 100 items in total) - this not only makes large collections impossible, but it forces the player to go through the very un-fun process of "which items do I discard" every time they pick up loot from an enemy. It's only good for Survival games; most other games should opt for per-Item limits rather than overall inventory limits. "Charges" of your refillable Healing tools in
Dark Souls also effectively use this Plentitude + Hard Caps concept.
Final tip - any time you are going to introduce rare items (or rare anything-else) to your game but you are also going to encourage your players to
use those rare items, give the player some kind of "Collection Book" that will show the list of all the items they've found along the way, even if they don't actually have the item in inventory anymore. This will satisfy a bit of the Accumulation/Collection itch without making the player feel like they have to hoard everything they find.