屡试不爽

屡次试验都没有差错。爽：差错的意思。易误解为“没有成功”。

清·蒲松龄《聊斋志异·冷生》：“言未已，驴已蹶然伏道上，屡试不爽。”

[be found officious as often as it is tried;be often tried and often succeeded;effective every time it is used] 每次使用都有效;多次试验都不错