Awesome topic bella989! Good boundaries really help with problems like this, as do "action plans." I use both.
I know, on pretty much a constant basis, what my financial and time limit boundaries are, as I'm careful to keep up on my budget and calendar. This way I know in advance what I can and can't agree to.
I've found that most people won't get upset when you say no when they know that you genuinely care. By this I mean, they sense that you're not trying to sidestep them but simply can't help. When someone asks me for help and I can't give it, I'm careful not to avoid them, or say yes and keep putting it off with excuses. Instead, I make it clear that I can't help, but then I stay engaged in the conversation and help them find ways to help themselves. I help them think through all the other resources (people, organizations, etc) available to them, as well as creative ways they can come up with money or whatever else they need. That way, I still care and am still helping- without overextending myself.
Keep in mind, too, you do NOT have to explain your reasons why you can't help. Explanations give them room to argue. If you say you have inlaws visiting this weekend, they'll suggest next weekend. And so on. Simply say "life is too crazy right now." If they question that, say "I'd rather not get into it. Let's focus on your problem and see if you can find some other way to fix it." It takes you right out of the spotlight and helps them help themselves!
Good luck!