There is a Danish song going: “Your mouth says no, no, but your eyes say yes, yes”. Very often, even though you do your best to speak in an assertive way, other things, like your eyes or your voice, will betray you. Let’s take a closer look.
First of all, do like grandma told you: Stand straight. Stand in a neutral way, arms along the side and on your two feet, meaning without any external support. It might seem pretty aggressive, if you are leant up against the wall in a relaxed way: I am totally relaxed, and I don’t care much about what you have to say to me. Don’t put your hands in your pockets and no crossed arms or legs. Just stand.
Don’t stand too close to the person, about 1 metre (40 inches), is fine. If you are too close, it could be perceived as aggressive. If you are too far away, it could be perceived as submissive. Beware, though, that there is some difference depending on the status of the person. 40 inches are fine if you are speaking to a colleague, but it is too close if you are speaking to your boss. Keep your distances.
Next, look the person in the eyes. Don’t stare and do dare blinking, but look the person in the eyes. Don’t look down. You may look a bit away from time to time, just not down.
Don’t smile. If you smile while talking, it can be understood as aggressive, like you are making fun of the other, or submissive, like you are trying to soften the other person.
Watch your voice, use a neutral tone of voice. Speak loud and clear so you are heard but don’t sound like a burglar alarm. There are many possibilities in between the two. Do not sound like you are asking a question or permission to speak your mind. Very often, if you feel unsure, you might let your voice go up at the end of the sentence, like you are asking a question. This is not a good idea, when you want to sound assertive.
Control your movement as much as possible. Don’t play with your necklace, or your hair or your ring or a piece of your clothing. This shows you feel very unsure.
There may be some things that are out of your control. For example, you might tend to blush. In this case, just thinking of not blushing can make it worse. So instead of fighting it, accept it as a fact. Think that you do tend to blush when you are speaking your mind, and that this is not the end of the world. It is not a catastrophe.
By accepting you are blushing from time to time, you will reverse this tendency by blushing less and less for each time.
Before you go on with your message, prepare yourself. Write on a piece of paper exactly what you are going to say, then repeat it until you can say it fluently. Don’t think about your body language or your facial expressions at this moment. Just concentrate on what has to be said.
When you are able to say the message fluently, stand in front of a mirror and work on your body language and facial expression. Don’t be to hard on yourself but do notice your errors and correct them. Don’t be too disappointed if you can’t do it correctly right away. It is very logical that it will take you some time to learn a new behavior. Most changes take some time.