Price the bench spot
Every prospect stash costs something. In shallow leagues, the cost is often flexibility. In deeper formats, the cost may be a streaming pitcher, speed bench bat, or injury replacement.
Before adding the prospect, decide what job that bench spot was doing.
Demand a path
Upside without a path can sit on your roster for weeks. The best stashes have a realistic opening: a shaky major-league role, a team need, or a player whose skills are close to forcing the issue.
The path does not have to be guaranteed, but it should be explainable.
Know what category you are buying
A stash is stronger when you know what it can change. Power, speed, strikeouts, ratios, or playing time should be part of the reason.
If the pitch is only that the player is interesting, the add may be better left on the watchlist.