Prevention is mostly about how you buy at the start. The instinct is to order deeply because the unit price improves at volume, and for a business with no sales history that discount is frequently the most expensive money you will spend. Buy the smallest quantity a supplier will accept for anything unproven, accept the worse unit economics, and treat the difference as the cost of finding out whether it sells. Buy deeply only into things with a demonstrated rate.
Catch it early by tracking how long each item has been sitting rather than only how much you hold. A simple aging view, showing units on hand and days since the last sale, surfaces problems while they are still small. Set a threshold appropriate to your category, review it monthly, and treat crossing it as a decision point rather than something to think about later.
When something is genuinely dead, act quickly, because the value only goes down. Discount it decisively rather than in cautious increments, bundle it with something that does sell, offer it to a liquidator, or donate it and take the deduction. All of those beat holding. The instinct to wait for full price on something that has not moved in a year is the sunk cost fallacy wearing a business justification.
The uncomfortable discipline is treating the original purchase as spent regardless. What you paid is irrelevant to what you should do now. The only question is which option recovers the most cash and space soonest.
Watch the ratio between what you hold and what you sell rather than the absolute quantity, because that is what tells you whether inventory is turning. Stock sitting for three times the period it takes to sell comparable items is heading toward dead regardless of how much of it there is, and catching that at three months rather than twelve is the difference between a discount and a write off.
Account for it honestly rather than carrying it at cost indefinitely, because a balance sheet listing unsellable inventory as an asset misrepresents the business to you as much as to anybody else. Writing it down is uncomfortable and it produces an accurate picture of what you actually hold, which is what any decision about buying more should be based on. Your accountant can advise on the treatment, and the discipline matters more than the method.