Here comes a big word, a wonderful idea apparently on the part of the United States ! In the field we are discussing, as Personal Assistant, assertiveness presents itself under a whole other definition: the art of diplomacy. This art, as it is one, is characterized by two essential principles: the respect of others and the respect of thy self. Is assertiveness something you can learn? Can you learn respect? Some essential techniques, such as being able to say no without causing frustration to the person you are speaking to, or to not diminish thy self in a delicate situation are techniques calling for a thorough work on thy self. For example: you feel guilty when you need to say refuse a task (such as lending your car to your buddy). The ability of saying no while clearly arguing and focusing on impossibility rather than the refusal in itself will allow you to come through as being diplomatic about your decision.
To express myself more clearly: your friend asks for your car and you answer : it is not that I don’t want to, but I really can’t ! If he insists and asks why, you can answer: because I need it! And if he insists still and calls you on as being a poor friend, you can simply answer: yes, I will admit that I can sometimes be. You will then keep your self-respect and the respect of others. The act of acknowledging that you can be does not mean you are, but it will allow you to cut short to the argument. In fact, you just had the last word.
This being said, it is not always easy to refuse as guilt can sometimes take over. However, I can affirm this from experience; the ability to say NO will not only bring you a better image of yourself but also a higher respect from everybody else.
As a personal assistant, this principle will allow you to keep your notoriety with all people you will be asked to work with.