Saying no is so damned diantre difficult. I know...
That's why I have some suggestions for you, or at least one.
Your best bet is to IDENTIFY WHEN you want to say no.
Sometimes you think you can't say no, because you have actually forgotten up to the fact that you can say no.
You think you should say yes so hard, or you want to please so hard that you ignore the signs that tell you you want to say no.
Any "shut yourself up" habits you may have such as eating out the fridge, smoking, or anything that is related to swallowing back what you have to say will be an indicator.
So what's yours?
Once you've got it identify, are you, or are you not in a position that permits you to say no, right now?
For example, are you struggling to answer no to an offer to move forward in a new position (in which case, you might have the chance to say no, and choose to stay secure), or to enter a relationship, or even just to go out for dinner?
In which case would it be ok to say no and still feel safe concerning your vital needs (food, shelter etc) ?
Saying no to an offer IS ok.
Add to it that saying no to an offfer made after you've asked for a favour, or for advice, or help still IS ok.
You can ask and then when offered inreturn go for "No, thank you. I have changed my mind".
(Just make sure you don't apply that to romantic relationship, because that can hurt deeply. Keep it for business sake).
In any cases, please bear in mind that
YOU DO NOT SUSPECT WHAT OPPORTUNITIES A NO CAN BRING ALONG.