As always, I suppose the happy serenity answer is an inside job, and is driven by what motivates me. If I say yes to something externally, I damn well better mean yes on the inside too so that I am not in conflict with myself. That's where I get into trouble and get uncomfortable, when I agree to do something to get someone off of my back or to appease them, but it's nothing I really want to do at all, and the only reason I am doing it is because someone guilted me into it. That's sickness on my part, and peace with a pricetag.
I give in to pressure too easily. I want to remember to slow down, and if I feel pushed and defensive, I need to buy myself time for retreat and regroup. At the very least, then whatever I decide is my decision and not a reactive agreement to deflect a combative situation. Today, I will pause when I feel any heat from others. I will breathe and consider and be sure to speak quietly and not get all hooked in and explosive. If something is asked of me and I am not sure, I will say so, and express my need for time to consider and reflect. Then I can come back to it thoughtfully and be in accordance with my whole self, and be clear about where I stand on the issue, and why, and stand up or surrender with understanding instead of weakness.
I commit to myself to not be reactive and popped about emotionally by others like a pinball. If I feel attacked, or am unsure, I use the option to retreat and consider thoughtfully before expressing where I stand.