It’s not just what you say, but the way that you say it
What you say
If you are to get a message across effectively, you must be direct and accurate. Use short, punchy sentences and try to summarise whenever possible. If people have to identify the important information from a long dialogue, they will eventually give up and your message will be lost. 
Consider the person you are addressing and the time and place. For example, you wouldn’t use a team meeting to speak to an individual about their poor performance.
Taking ownership of your message shows assertiveness, e.g. stating: ‘in my opinion … ‘. This is not confrontational, as you are offering an opinion rather than stating fact. People are less likely to respond defensively or aggressively to your opinion and are more likely to take your view into consideration. 
Tackle the problem
Avoid confrontation by tackling the problem and not the person. For example, ‘Why can you never get anywhere on time?’ is a personal attack; while ‘Please make sure you are here on time’ is a solution to a problem.
Work on using clear, concise, constructive language to get your message across effectively. Choose words you are comfortable with. It is important to match your vocabulary to that of the receiver. If you use language they won’t understand such as technical words and jargon you will only distance yourself from the person with whom you are trying to communicate.
Stay on track
During communication, things may happen to knock you off track. For example, someone might disagree with you. In this case, you will need some time to reconsider your views and prepare an assertive response. You can gain valuable seconds by using starter words, e.g. ‘Well … or I see … ; asking for clarification or checking your understanding; or asking for time to think about what the other person has said.
The way you say it
Use your voice to full effect. Don’t shout but add quality to your voice. To do this, you can apply a breathing technique used by singers: Put your hand on your stomach and breathe deeply so that your hand moves in and out. This is abdominal breathing, which allows you to access the lower/richer end of your voice range. If you use abdominal breathing while speaking, you can take in more air and thus talk for longer without pausing.
A monotone delivery is dull. A good delivery flows smoothly and has inflection. Emphasise certain words to add meaning to your message. Try not to accidentally give the wrong meaning. For example, ‘I can’t have a meeting with you tomorrow’ may be seen as offensive, whereas ‘I can’t have a meeting with you tomorrow’ is more reasonable.
Your posture can communicate confidence and control. For example, when standing, keep your head up and maintain an open stance, arms hanging loosely, hands open and relaxed. This shows control over yourself and the situation. Hunched shoulders show passive behaviour. Crossing your arms can be defensive, and angling away from a person indicates that you want to be somewhere else.
Your facial expression should be appropriate to the message you are giving. For example, a smile can be disarming, but is inappropriate if you are giving bad news.
Good eye contact demonstrates respect and openness. Avoiding eye contact, on the other hand, can be a sign of nervousness or dishonesty.
Respect another person’s personal space as standing too close will make them feel uncomfortable.
People who listen but are easily distracted are not communicating effectively. Good communication includes taking on board the opinions and rights of others. You should try to read the messages others give out. For example, if they seem distracted, you may offer to arrange another time to talk.
Make sure anything that the receiver can see is in order. This will give you the appearance of being prepared and confident.
Confident people don’t look nervous. If you get nervous easily, then you might like to try some relaxation techniques, such as deep breathing, before entering a situation that makes you nervous.
 Jan Ferguson, Perfect Assertiveness (Random House Business Books, 2003).
 Sue Ferguson, Develop Your Assertiveness Second Edition (Kogan Page, 2000).