Lather, rinse, repeat

Lather, rinse, repeat is a phrase or idiom roughly quoting the instructions found on many brands of shampoo. (Often the phrase is shortened to "Rinse and repeat".) It is a humorous way of pointing out that such instructions if taken literally would result in an endless loop of repeating the same steps, at least until you run out of shampoo. It is also a sarcastic metaphor for following instructions or procedures slavishly without critical thought.

In the book The Plagiarist by Benjamin Cheever, an advertising executive increases the sales of his client's shampoo by introducing the word "REPEAT" to its instructions.