And if they persist, just keep saying "Sorry, I can't, but thanks for asking!"  If they don't quit, they are overstepping their bounds and you have every right to put your foot down.  Granted, that's tough for people-pleasers (like me), but it's a skill you can certainly learn.  But the positive no is by far the best way to frame a "no" without feeling like you need to make excuses or be rude.  And with normal people, it works.  Not with narcissists, but you can still use it, you'll just most likely have to put your foot down at some point with them.  

So, my neighbor who oversteps by asking me (or really anyone) to watch her dog for two weeks to a month at a time, I just told her I couldn't watch her dog for a month in January.  And this is how I phrased it: 

"Hey there!  We want to say that we feel very honored you'd trust your baby in our care for a whole month, as he's a very good boy, but we don't think we'll be able to watch him in January during your trip.  We have so much on our plates and I don't see that changing anytime soon (in fact, things will be getting busier) and that length of time is just too much for us.  But we are always more than happy to watch him up to 1-2 days at a time whenever you need us to.  We enjoy his company!" 

First of all, I said "we" throughout the entire thing because she wanted my kids to do it, too, so now she realizes I mean everyone in my house, not just me.  Second, I added "and that length of time is just too much for us" so she gets that asking me for 30 days of going to her house several times a day, plus cooking a meal for him each day, is just too much to ask for.  Third, I added a concrete amount of time I am willing to watch him for.  So there is no confusion.  This says "do not ask me for any amount of time more than this".  Granted, will she remember that?  Probably not.  But still.  I made it clear so we can come back to it later if need be.  Fourth, I ended on both an alternative that I will do for her and a nice sentiment.  The idea with this is that a) they don't get offended (though some narcissists will anyways), b) you give them a clear and concise no, so they can't come back and battle your excuses (because a narcissist/toxic person will: they will look for a weak link in your argument until they are blue in the face) and c) the positivity you give them will make them feel like your no isn't really a no at all, but a nice and kind retort to their question.  Its psychology.  

But not only that, everything you fear about telling someone no is alleviated with this technique.  Fear you have to come up with a good excuse?  Not needed.  Fear someone will tear apart your excuse?  Won't happen.  Fear someone will get upset?  Less likely to happen.  Fear that you'll be seen as rude?  Won't happen.  Fear that you're actually being rude?  You're being the exact opposite.  Fear of anything else?  This technique will make it so you have nothing to fear.  And if someone does get upset, because all they hear is the no and not the kind words?  Or they keep hounding you anyways?  It's no longer your problem.  Because just by the sheer fact you crafted this "positive no" and sent it to them (or said it), it will give you more confidence to stand your ground.  

The book has everything you need to know.  If you're like me, and you're "guess culture" (check out this video if you don't what the difference between "ask culture and' "guess culture" is) or you're a people pleaser who doesn't know how to say to people (also like me), then I highly recommend this book.  Which you can buy below.  If you purchase through my link, I get a tiny portion of the sale at no cost to you.