Offering constructive criticism can be a powerful way to evoke a positive change in a person's behaviour. The problem is, like most things to do with construction, you have to take great care when laying the foundations. Otherwise you just dig a hole that you can't get out of.

For instance, telling somebody they're a lazy good-for-nothing whose total lack of knowledge, skill and endeavour means they will never amount to anything in life, is not likely to encourage them to try harder. I still have no idea why my Headmaster wrote that on my report.

The bottom line is constructive criticism should encourage whereas any other form of criticism is destructive and usually just highlights a problem, instead of offering a solution. So, after years of providing constructive criticism to employees, and managing to dodge flying fists and avoid temper tantrums, here are my thoughts on how to do it effectively.

First you have to build trust and show that you care because nobody willingly takes advice from someone they think doesn't have their best interests at heart. That's why double glazing salesmen have such a tough time. So make sure you praise them whenever they do something good. People like their efforts to be recognised and, over time, this will help them to consider your comments valid, relevant and welcome.

Now suppose they do something that isn't very clever - that causes you to lose money or face with a client, for example. Often it will be the result of a genuine mistake rather than a malicious error and the chances are they know what they did wrong. Rather than admonish and criticise the person, you need to sympathise and encourage. Concentrate on what they need to do to put it right rather than what they did wrong.

However, if it turns out it was a malicious error you should string them up by the thumbs and take a long lunch. That can work too.

But in general, if you build up a good relationship people will trust and respect your judgement. They'll expect praise when it is due but also be prepared to accept constructive criticism too.

And when something negative happens, a degree of tact goes a long way. A technique I often use is to get them to acknowledge the mistake by asking them what areas of their work they can improve or develop. If they introduce the subject it's easier to talk about.

So be honest, be fair, praise when it's due and always try to put a positive spin on a negative situation.

Philip Hesketh is a professional speaker on 'The Psychology of Persuasion' and author of "Life's A Game So Fix The Odds."