It is highly likely that within the past 24 hours you have done something that economists maintain makes little sense: you voluntarily increased the price of a service by adding a gratuity.

Tipping service workers is widespread in the hospitality industry; in the United States, where remuneration in 33 professions includes tips, the annual bill for tipping in restaurants comes to $26 billion (pound;13.9 billion). If you depend on these extras to boost your income, there are a few tricks that will help you maximise your profits. For example, research highlights the importance of standing out from the crowd. Customers then perceive you as an individual rather than a faceless member of staff.

The results of one study confirmed that a waitress who wore flowers in her hair increased her tips by 17 per cent. Wearing something unique or unusual personalises you in customers’ eyes. But take care not to wear anything bearing political, religious or otherwise controversial messages.

Sunny weather puts people in a good mood and people in a good mood leave bigger tips. Psychologists reason that, because even the prospect of sunny weather elevates people’s moods, service staff who work where the weather is variable can increase tips by telling their customers sunny weather is on the way.

To test this idea, Bruce Rind of Temple University and David Strohmetz of Monmouth University, both in the US, asked a waitress at a mid-priced Italian restaurant in New Jersey to write a weather forecast - “The weather is supposed to be really good tomorrow. I hope you enjoy the day” - on the back of some of her bills, but not others. The positive forecast boosted her tip by almost 20 per cent.

The researchers say waiters can increase their gratuities using such psychological techniques as giving their names to customers, squatting next to customers’ tables, touching customers, giving after-dinner mints and encouraging customers to pay by credit card. Could teachers use any of these strategies with their own “customers”?

The psychology of tipping rests on the key assumption that the worker has some control over the quality of the service. The idea of variable quality at the worker’s discretion is a deeply troubling notion in professions such as medicine and teaching; after all, we rarely offer our GP a tenner after a check-up, and if we were buying brain surgery, for example, we would expect the best service to be included in the price, as we do with teaching. This suggests that we embrace the idea of tipping only for non-essential services.

This is illustrated by an intriguing finding from a study which found that a waiter who drew a happy face on restaurant bills significantly increased tips. Imagine, in contrast, a brain surgeon drawing a happy face on his or her bill, regardless of the outcome of the operation. The key point is that educationists often get obsessed with comparing teachers, ignoring the fact that there is bound to be variation in one teacher’s performance over time.

Increasingly, education managers are required to assess the performance of their staff. The reason this is so difficult might lie in the deeply ingrained belief that professionals such as teachers and doctors always do their best, all the time. As no one else does (which is partly why tipping exists), why do we believe it of doctors and teachers?

Dr Raj Persaud is a consultant psychiatrist at the Maudsley hospital and senior lecturer at the Institute of Psychiatry in London. He is a fellow of University College London, and author of From the Edge of the Couch published by Bantam Press, pound;12.99. Email: rajpersaud@tes.co.uk