Tip 1: Don't Even Bother Trying, Just Say No
When in doubt, say no. It's easier that way. Don't waste time and energy on unnecessary discussions or negotiations. Just be the one who says no, and be done with it.
Subtip: Be Prepared for the Backlash
People may not like it, but who cares? You're setting boundaries, after all. Be prepared for the drama, the tears, the passive-aggressive emails. It's a small price to pay for maintaining your sanity.
Subtip: Use 'No' as a Complete Sentence
Don't even try to explain or justify. Just 'no'. It's a beautiful thing, really. Try it out: "No, I won't do that." See how simple that is?