Remember when Nancy Reagan's response to the drug epidemic was "Just Say No". As Drug Strategies, a nonprofit research institute based in Washington, D.C. found out, just saying no doesn't work unless you first learn Refusal Skills.
The important thing about our ability to say no is that it keeps us safe and provides the opportunity for self-definition. As they say, your yes doesn't mean much, unless you can say no. But saying no in relationships can be particularly difficult. You may believe that saying no means you never get another chance with that person. You may believe that saying no is not OK because it is rude. You will hurt the other person's feelings and that will make you bad. Both of those beliefs can be self-fulfilling prophecies. However, both beliefs are erroneous.
Assertiveness skills training was developed in the late l960's and early l970's. The goal is to get a win-win situation. To be aggressive you count, the other person doesn't. To be unassertive, they count and you don't. To be assertive you count and the other person counts. You have to focus on compromises, building bridges and being clear. Being incisive means that you are clear and kind. Being decisive means that it is the end of the discussion. You may win, but you will ultimately lose. Through role-playing, values clarification and practice you can be "connected", but not swallowed up.