In response

To someone on FB asking how to know when to say Yes and when to say No:
The answer is always what feels genuine and true for you. It's not the No or Yes that's the point. The point is for you to learn the practice of knowing and being true to yourself. At first that requires somewhat arduous and very deliberate self inventory, and that takes time. So for a while always respond first with, "I need to think about it but I'll get back to you as soon as I can." (Or something along those lines)
Then consider the question at hand ("X") with the following questions: Do I feel inspired to do X? Do I realistically have time to do X? Am I realistically capable/have the skills to do X? Will it cause unnecessary stress to do X? Do I feel obligated? If so, is there a way to manage or adjust my time, the task or the other people involved in a way that will make this as positive as possible?
The trick is to discern between what is likely to cause only stress with little reward and an opportunity which may result in some stress but mostly of the life-expanding challenge kind.
I have fibromyalgia so for me nearly anything that's above and beyond my regular daily justgetthroughtheday stuff. That's why I had to learn this technique. Before, I would say No but to almost anything. Then I realized I was missing out on a lot of life. So then I started saying yes but I overdid it.
This personal questionnaire has really helped me have a better life. Last but not least - decide to be OK with saying No. It really is OKAY and your value as a person is actually increased by respecting your limits and happiness!
Hope that helps!