Assertiveness is a way of standing up for yourself. It is a way of making a point in an open and rational way and it's a way of showing others exactly how you feel at the moment. It is the direct opposite of being passive where you take little self- protective action. Aggressiveness generally involves standing up for your rights but at the same time undermining the rights of others. It is important that assertiveness be used at a time when the persons involved have equal power and status if at all possible. An example would be with a spouse, a close friend, or a member of your family. There are however, times where you can be assertive without equal power, for example, if you are a supervisor and it's your duty to manage the activities of others.
It is of course not a good idea to use assertiveness with people who have more control over you than you would have reciprocally over them. If you might have a lot to lose in the outcome of an interaction, it is best to take a more diplomatic approach.
How others experience an assertive message is determined by several behavioral factors. Most important are the contents of what you say, how you say it and your behavior when you say it. Closely watch your verbal and nonverbal signals because your spouse, friend or family members surely will! A truism in psychology is "the message sent is not always the message received".
Psychologists also know that a couple will benefit from a close examination of their manner of speaking to each other. For example, starting a sentence with the word "you" can be significant. When someone starts a sentence with the word "you", very likely a criticism of the other will follow. For example, "you are lazy", "you are not measuring up" and "you should make these changes..." It is far better to begin sentences in this situation with "I". For example you might say, "I feel this way..." or "I think it's time that we did..." Can you see how less confrontive the use of "I" is when compared to beginning a sentence with "You" under these circumstances?
Also helpful is the technique of avoiding the use of words that tend to exaggerate. For example "You are the worst at..." or "Your brother can do this 10 times better than you can!" In situations where there is an intense emotion, you can use what you say and how you say it to defuse and minimize any conflict that might be impending. An important technique in this area involves the ability to take a step back and observe yourself interacting. Once you develop this skill, you will then be able to use your assertiveness with just the correct amount of intensity for the circumstances.
When being assertive always keep in mind the difference between assertiveness and aggressiveness. Aggressiveness frequently will demean and undermine a relationship whereas assertiveness, when used correctly, can actually strengthen it.