How do I say no to people that ask me to do things and then guilt me into saying yes?

For what it's worth, I get this question a lot.

The problem with boundaries, is (1) the answer is one you don't want to hear and that (2) it takes time.

We aren't really born trying to look out for others' boundaries. We're born trying to keep to our own needs and desires. So if I want to go to the store, and you want to go to the park, it's only natural for me to REALLY RATHER we go to the store, and for you to REALLY RATHER we go to the park.  So teaching somebody else that our boundary is important to us and that we mean to keep it, requires being consistent. 

So for example:

• You: I think I'm going to sit this one out. I want to stay home today. 
• Friend: But it's such a great show! Please, you'll regret it!
• You: I'm going to sit this one out, I want to stay home today. 
• Friend: But you stay home so often, and I NEED a buddy!
• You: I'm gonna sit this one out, staying home is really what I need for myself. 
• Friend: But!!!......
• You: It's important for me to stay home
• Friend: But! more arguments!!
• You: I'm sorry you're disappointed, but I'm staying home. 

And really doing that until bedtime. The first 10 times will take a lot out of you, but the more they practice your boundary, the more they'll get it that "if they said no the first time, they'll probably say no the 50th". The problem right now is that if you 'cave' on number 7/ 12/ 33 so they think that all they need to do is ask 33 more times, and then you'll say yes. And since we all need to take care of our own needs, they'll be super motivated to try 33 more times, since then their need might be met.

Taking our own boundaries seriously is a lot of hard work, and takes a lot a lot of consistency. But it gets easier with every time, and over time gets much much easier. 

I wish you all the luck and growth with this. It's a hard task, but it's super worth it!