Before you decide to act assertively in a given situation, you have to decide if you can live with the consequences. Although assertive behavior usually will result in a positive response, some people might react negatively to it.
If you're planning to try assertive behavior, remember that the other person is used to your behaving in a certain way, and may be confused when you change your communication style. Why not tell the other person up front what you're trying to do? It helps to choose a peaceful moment for this.
An example of an assertive communication:
"I need to tell you something and I'd like you to hear me out before you comment. I've noticed that whenever we have midterms, you forget to clean your side of the room. I know you get anxious when you have to take exams, but you're not cleaning the room really frustrates me. Can we clean the room together to create less stress for both of us?"
Use assertive body language. Face the other person, stand or sit straight, don't use dismissive gestures, be sure you have a pleasant, but serious facial expression, keep your voice calm and soft, not whiney or abrasive.
Use "I" statements. Keep the focus on the problem you're having, not on accusing or blaming the other person. "I'd like to be able to tell my stories without interruption." instead of "You're always interrupting my stories!"
Use facts, not judgments. "Your punctuation needs work and your formatting is inconsistent" instead of "This is sloppy work." or "Did you know that shirt has some spots?" instead of "You're not going out looking like THAT, are you?"
Express ownership of your thoughts, feeling, and opinions. "I get angry when he breaks his promises." instead of "He makes me angry." or "I believe the best policy is to..." instead of "The only sensible thing is to ..."
Make clear, direct, requests. Don't invite the person to say no. "Will you please ... ?" instead of "Would you mind ... ?" or "Why don't you ... ?"
Broken record -- Keep repeating your point, using a low level, pleasant voice. Don't get pulled into arguing or trying to explain yourself. This lets you ignore manipulation, baiting, and irrelevant logic. Example: You are taking something back to a store that you know gives refunds, but the clerk first questions your decision, tries to imply that there's something wrong with you because you changed your mind, tells you that she can only give a store credit, etc. Using the broken record, you walk into the store and say "I decided I don't need this and I'd like my money back." Then no matter what the clerk says, you keep repeating "I decided I don't need this and I'd like my money back." If she doesn't get it, simply ask to speak to a manager and say the same thing.
One of the most common problems in communication is caused by trying to read people's minds or expecting them to read yours. If you want people to respond to your ideas and needs, you have to be able to say what they are, and say it in a way that will make others want to respond appropriately. Do you remember the self-efficacy part from the beginning of this piece? The belief that if you do something in a particular way, you will be effective? Even if you don't believe that now, but you muster your courage and try some of these techniques in situations that are not extremely threatening, the results will probably be so encouraging that you will begin to believe in your effectiveness.
If it's really scary to think about being assertive, try it first with people you don't know. Think of someone you know who is assertive and pretend you are that person. Once you become comfortable with assertive behaviors in less threatening situations, you can crank it up a notch and use it all the time. When assertiveness becomes a habit, you will wonder how you ever got along before you started using it. After you've become truly assertive, you probably won't need to use these techniques very much.
As people practice assertive communication, you can almost see that little spark of self-respect glimmer, flicker, take hold, and burst into flame. People can sense it when you respect yourself, and they will treat you with respect. And that is the ultimate goal of assertive communication.