You're not alone. Many of us find it hard to speak up for what we want, to put views across in large meetings, and to disagree publicly with others. But you can break the cycle by practising a few simple tricks.

Start by analysing why it is that you don't speak out. Is it fear that makes you clam up? Are you afraid of looking stupid, or of your manager getting angry with you? If it is fear that strikes you dumb, you must ask yourself two simple questions, and answer them honestly.

The first is, 'What is the worst that could happen?' Will you lose your job, will you permanently damage your reputation, and is it a matter of life and death? The answer is probably 'no' on all counts. Then ask yourself, 'What is the best that could happen?' You might show yourself to be an ideas person, and that you have a lot to contribute. Your manager might perceive you as a stronger person than he or she previously thought, and admire your directness.

Like most things, assertiveness has to be practised. But don't start by volunteering to give a presentation to the Board - begin with non-threatening situations. Go into three shops and try on racks of clothes without buying any. If you are always the person who organises the group outings for your friends - but secretly you resent it - make sure you don't organise the next. Are you always expected to do the 'coffee run' in the office? Next time a colleague passes you the tray, gently suggest it's someone else's turn. Resist the temptation to say you'll do something, when you know you don't want to.

Then begin experimenting with voicing your own views. Resolve that the next time someone says something you don't agree with, you're going to disagree. Just take a deep breath and say what you honestly think. Do it without whining or pleading, but in a direct manner. Choose your language carefully. Telling a colleague their views are rubbish will probably end in a full-blown confrontation - not surprisingly. All you need say, in a measured way, is, 'That's interesting, because I feel quite differently. I don't agree that …'

Part of assertiveness is showing respect for other people, just as you hope they will treat your views with respect. Assertiveness is the knack of calmly standing up for your own rights without violating the rights of others. It comes from having the confidence to believe that your views are as valid as anyone else's, and understanding that you have as much right to express your opinions as others. Assertiveness is not table-banging aggression, and the two should not be confused.

One of the most difficult challenges for non-assertive people is saying 'no', especially in the work context. It's easy to assume you'll make someone angry if you say 'no', or that they'll think badly of you. Bear in mind that you're 'allowed' to say 'no'. You don't have to feel guilty about it.

If you're more comfortable doing so, offer a reason - 'No, I won't be able to do that because I'm working on an urgent report.' If it's a question of balancing your work priorities, put the onus back on your manager. 'Yes, I can do that, but obviously I won't be able to finish the Morgan Morgan report. Which would you prefer me to finish first?'

It sometimes helps to offer another solution, such as volunteering to do it at a later date, or suggesting someone else who may be able to help them out. If they keep asking, stick to your guns and be determined not to give in first! Use the 'stuck record' technique, where you politely repeat the same message until it gets accepted. 'I'm sorry I won't have time this week,' and then, 'No, I'm afraid this week is absolutely crammed for me, so I won't be able to fit that in,' and so on.

If you're asking your manager for training, or a flexible work option, or time out to do voluntary work, remember that it's not enough just to ask assertively. You need to put forward a business case. This means, submitting the reasons why the business or department will benefit, and not just you. It's crucial to be up-front about the time commitment you're considering, or the changes in routine that your project would entail.

It's also key to demonstrate that you have thought in advance of the potential problems, and have devised solutions. If you want to volunteer at a local special school twice a week and need two longer lunch-hours, work out not only how you will put that extra time back, but how you will cover your department when you're out of the office.

If you're persuading your manager to pay for an external training course, stress all the benefits to your department or firm that the course will bring. Will it make your department quicker, or more efficient? Will you learn a new skill which you can pass to others? If you put forward a solid business case to back up your request - and you ask professionally and calmly - you not only stand a better chance of having your request accepted, but you will make an extremely positive impression on your manager.

So start practising your assertiveness skills today. You probably won't crack it all first time, but be patient and you'll soon be letting the world know what you want in such a calm and irrefutable way that no-one will be able to refuse you anything!