If a river on a summer afternoon does nothing for you, use something plainer. Counting the traffic from a porch, or standing outside a shop counting how many people walk past, does the same job.

What the research says

In a randomised experiment with 62 students, training people to use planned self talk protected their selective attention after their self control had already been worn down. What was prepared in advance was the wording, and the wording was what carried the attention. It was a small laboratory study, so it supports having a phrase or image ready rather than telling you which one to pick.

Strength of evidence: one line of research. A single study, or a theoretical account.