There is a team at my company that works on a particular project. Several members are very well-versed in my area of expertise, but ask me to to their harder evaluations. They also ask for help close to deadlines, forcing me shuffle other work around to accommodate them. I did so during my vacation, but said I would only give them 2 hours. I gave them 3.
Today they asked me to finish the evaluation* in 1.5 days. I am completely over my head in work through mid-July at the earliest so I told them no. It felt good.
*which was never promised
Cedar Waxwing's 365 
Yay!!! Good for you. It is surprising how many people will take advantage of us if we let them. (And then they get promoted and become our bosses.)
Yes.
Learning to say a solid no and a real yes is the struggle of my whole life.
Good for you!
The first time is the hardest.