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Assertiveness is the key to standing up for yourself. It isn't just a cliché, it's a bona fide means for improving your chances of getting what you want and for being heard properly.  Being assertive enables you to express your wants, needs, and preferences in a way that shows you're prepared to stand up for yourself while still respecting the other person. It involves being open and honest about your thoughts and feelings while trying to work towards a mutually satisfying solution. When asserting your feelings and opinions, it's recommended that you use "I" statements, rather than "you" statements, as this is less accusatory and will prevent the other person from going on the defensive. For example, instead of saying "you never ask for my opinion", say something like "I feel ignored when you make decisions without me".   Assertiveness is, for the main part, a learned skill, so don't feel bad if it doesn't come naturally. There are many excellent books and courses on assertiveness training available. You might like to start by reading the classic When I Say No, I Feel Guilty, by Manuel J Smith, and Your perfect right: A Guide to Assertive Living, by Robert E. Alberti. See also How to be assertive and Communicate in an Assertive Manner. Learning how to say no is one of the hardest but most important ways of standing up for yourself. If you tend to be a "yes" person who never wants to let anyone down, you risk becoming a door-mat who everyone walks all over and takes advantage of.  For example, if your boss keeps asking you to work late when your co-worker has no problem skipping out the door at 6 pm, it can be very difficult to say no. But if this extra workload is putting your personal life and relationships under pressure, you need to put your foot down. Don't put someone else's needs above your own -- learn to say no when necessary. Learning to say no will help you to stand up for yourself with friends as well as people who intimidate you. Think about that friend who keeps borrowing money but never pays it back; assertiveness will enable you to ask for that money back and to say no next time, all while maintaining your friendship. People may be taken aback at first, but they will learn to accept your new-found determination and may even come to respect it. The manner in which you stand, walk, and sit makes a big impression on people. Positive body language can be used to garner respect, agreement, and trust, while negative body language (slouching, trying to shrink away) is practically an invitation to being pushed about.  Using open body language shows people that you are self-assured, confident and not to be messed with. Open body language includes leaning forward, making eye contact, standing with your hands on your hips and feet apart, using slow and deliberate gestures, facing your heart to people when you meet them and uncrossing your arms or legs. Closed body language, on the other hand, sends negative signals and could leave you open to attack. Closed body language includes crossing your arms, clenching your hands, using fast and evasive gestures, fidgeting, avoiding eye contact, and turning your body sideways. For many shy people, standing up for themselves is not something that comes naturally, but that's okay. All you need to do is practice -- soon you'll become more confident and more assertive about making your voice heard.  Sometimes you may fail to stand up for yourself simply because you cannot articulate what needs to be said at the right moment. Take the time to write out good responses to difficult situations and practice them with a friend using a timer. Have your friend pretend to be a difficult or intimidating person who showers you with put-downs. Put on the timer for about 2 minutes and respond away! Keep doing this until you get the hang of it. You can also practice standing up for yourself in small, everyday situations. For example, rather than quietly accepting the wrong coffee when the barista gets your order wrong, learn to say "excuse me, I asked for non-fat milk. Could you please make me another?" Soon you'll have the confidence to tackle bigger, more important issues! Another aspect of standing up for yourself is trusting your instincts about other people and learning to act on them. For example:  If another person is bringing you down with their negativity, don't hang around them; start to politely but firmly distance yourself. You don't owe difficult people any explanation as to why you're spending less time around them. Avoid bullies, negative Nellies, and sarcastic Sams. You don't gain anything from being in their presence and you're not doing them any favors by putting up with their nonsense or rewarding them for bad behavior. Remember -- keeping away from sources of discomfort and trouble is not running away; it is an important part of learning to stand up for yourself because it demonstrates that you won't let nonsense and nastiness impact your life.
Be assertive. Learn how to say no. Recognize your body language. Practice standing up for yourself. Stay away from negative people.