Start with a reason for every spare
A shelf of unidentified belts and filters is not useful resilience. Begin with the correct manuals and service information for the machines you own. List items you are likely to need, then ask a competent supplier which are appropriate to hold and how they should be stored.
Separate routine consumables from components that need specialist diagnosis or fitting. Owning a spare does not mean you are competent to install it.
Record identity before quantity
For each item, note the machine, part number, specification, supplier and purchase date. Keep packaging or a clear label so a future user can identify it. Where one part fits several machines, record the confirmed applications rather than relying on memory.
Avoid substituting an item because it looks similar. If a part number has changed, retain the supplier's confirmation of the replacement reference.
Balance delay against waste
Consider how often the item is used, the consequences of waiting for it and whether it deteriorates in storage. A service consumable needed at a known interval may be easier to justify than an expensive component bought for an unlikely fault.
Ask about shelf life and storage instructions where relevant. Do not accumulate fluids, batteries or other materials without the storage arrangements they require. Dispose of obsolete items through an appropriate route rather than transferring unlabelled stock to another machine.
Use a small stock record
Keep four simple fields beside the shelf: item, quantity, minimum level you have chosen and last movement. Update the record when something is used, not at the end of the season when nobody remembers.
Before placing an order, check planned servicing and what is already held. Mark items reserved for a booked service so they are not unexpectedly consumed elsewhere.
Review after real experience
At the annual machinery review, identify stock that was useful and purchases that sat untouched. Adjust the list accordingly. HSE's work-equipment maintenance guidance provides the broader context: maintenance arrangements and competence matter more than the size of a spare-parts collection.