Pick up a newspaper, watch a movie or the TV news and you'll get the message, loud and clear, that anger is a Very Bad Thing. Road rage, desk rage, trolley rage – if we believe the media then anger is scary, destructive and something we should avoid at all costs.

In fact, the problem is not anger, it's aggression: the unhealthy distortion of a perfectly natural emotion. Like sadness, fear or love, anger is neither good nor bad, it just is.

The problem comes when you express anger in one of two dysfunctional ways: you become aggressive and struggle to control your angry outbursts, using threatening language and behaviour to exert control over others.

When you’re seething inside

Or passive, becoming scared of anger – both your own and other people's – meaning that you give away your power, struggling to impose yourself on the world or fight back when bullied, even though you might be seething inside.

Both distortions of anger can be extremely damaging. The first damages those around you, as you attack or intimidate them. Eventually, of course, if you keep lashing out you cause problems for yourself too – when your destructive behaviour gets you sacked, divorced or arrested.

If you belong to the second group, you mostly hurt yourself – bottling up your anger causes stress, anxiety, depression, cardiovascular disease, high blood pressure, stroke... the list goes on.

Assertiveness, not aggression

Either way, you need to learn how to express anger cleanly and healthily – responding with appropriate, proportionate anger in situations that demand it (a rude co-worker; a bullying spouse; a salesman trying to rip you off). Anger gets a very bad press, but if felt and expressed healthily it's a great source of power and strength. The whole evolutionary purpose of anger is to protect yourself and those you love from attack. So the key is to be assertive, not passive or aggressive.

Some of humanity's greatest leaders have been the living embodiment of assertiveness, like Gandhi or Martin Luther King. Both practised non-violence, but transformed the lives of millions of people through sheer determination: they could not be bullied or intimidated, and maintained great dignity in the face of brutality and aggression.

If you would like to be more assertive, next time someone upsets you try this exercise:

1. Get the person's attention. This won't work if they're reading the paper or fiddling with their Blackberry.

2. Describe the behaviour you found difficult. Do this without personalising it or making accusations. Just stick to the facts: 'In that meeting you kept interrupting, talking over me and dismissing my ideas.'

3. Tell them how it made you feel. Use 'I statements' and take responsibility for your feelings: 'When you constantly interrupt me I find it frustrating and annoying.' Avoid emotions like anger, hurt or jealousy, because these will undermine your attempt to be assertive.

4. Check your interpretation and ask them to respond. Your interpretation of events may have been completely inaccurate, so it's very useful to check them against reality. 'Do you think you interrupted a lot? Did you feel dismissive of my ideas?'

5. Listen to the other person's response. Try to be non-defensive (this can be hard, but it will really help) and expect their interpretation to be different than yours. That's OK – they are entitled to their opinion, but you don't have to accept it.

6. Tell them how you would like it to be. This means expressing preferences ('I would appreciate it if you stop interrupting me in meetings'), not demands ('I'm sick of you interrupting – don't ever do it again!').

Of course, this is a bit of a lengthy process. Once you've got the hang of it you can boil it down to a much shorter exchange. And if you do this, regularly, you'll be amazed at the difference it makes to the way you feel and the relationships with your partner, family and colleagues.

Because it's so important, I would like to repeat this: expressing your anger healthily does not mean attacking anyone else, either verbally or physically. That is both unhealthy and destructive for all concerned. Managing your anger is about finding ways to be more assertive, expressing what you really feel and need without lashing out or stuffing your angry feelings.

Warm wishes,

Dan