Handout about assertiveness
Being assertive can be key in expressing yourself properly within a team. If you feel insecure about your assertiveness, it is useful to read through this handout. It explains the concept of assertive behaviour relative to interest in others and self-interest, as visualized in the figure below.
It deliberately explains how you can politely refuse a request, which may be difficult to some people. Saying no can been done in an assertive way by for example paraphrasing the request before you say no, that is usually a sign of respect. Giving a reason is not always necessary, making up excuses is something you should try to avoid.