Having this kind of mindset - a commitment to excellence, understanding every act of service as a testament to God's glory, living freely and giving generously of yourself - it helps you to figure out what the things are in life that you should say yes to. And perhaps most importantly, what the things are in life that you should say no to.
This has been a big debate for a long time. Is it ever "nice" to say no to someone or something? Is it ever an act of good, Christian faith to deny someone else their request? Are we "allowed," whatever that means, to ever say no?
On one hand, we have a Jesus who tells us that when someone asks for our jacket, we should give them our shirt, too. We have a Jesus who says that when someone makes us go one mile, we should go two. We have a Jesus who says that we should go above and beyond for others. And certainly, He is right.
But it's a misinterpretation of Jesus's words if we use them to claim, "Say yes to everybody that asks anything of you. Never say no to anyone or anything."
At another point in the New Testament, we are told that if it's not against your conscience to eat "unclean" food, then eat it. But if it is against your conscience to eat it, then don't. And don't condemn someone else for what their conscience permits (or doesn't). So clearly, there is something holy about saying no sometimes.
Here is, I think, one criteria for deciding when to say no: if you can't do something well, then say no. If you can't do it in such a way that it brings glory to God, say no. If you can't commit yourself to excellence, say no.
Now, there are a number of different reasons why you might not be able to do something well, why it might not be glorifying or excellent. The first reason, of course, is that maybe you don't know how to do that thing, whatever it is. If someone asks you to come help them put in a new water heater and you don't know how to do that, then the faithful thing is to say no. Sorry. That's not something that's in my skill set. Because the truth is that if you don't know how to do something, you can't possibly do it well. And when there's water leaking all over the basement, nobody is going to be thankful for your help. Nobody's going to be thinking what a good friend you are. So if you don't know how to do something, say no.
If you don't have time to do something well, then say no. Maybe you're very skilled, but you're also very busy. Maybe you have a thousand other things on your plate right now; it's just a frantic season for you. Maybe you know that the task someone has asked you to do is realistically going to take seven hours; don't try to squeeze it into three. Don't try to pencil out time between two other commitments you've already made. If you do, you will fail all three of those tasks - the first will be rushed to get to the second, and the third will suffer from the exhaustion of the first two, and if you didn't start with enough time to begin with, things will either remain undone or will be poorly done/slapped together. If you don't have the time to do something wholly, then say no.
If you're not willing to make a commitment to a project, then say no. Here, we're talking about the kinds of things you take on just to cure your own boredom. You do them because they're fun for you, but it's the act of doing it that's more important than finishing the job. You get so involved in the enjoyment and satisfaction that you get from it that you forget that someone else is waiting on it to be done, that someone else has plans for its completion. In a slow season, a contractor took on a couple of jobs, and it took him more than three months to finish putting siding on one house. Because he liked having a job to work on, because he liked having somewhere to go in the mornings, because it was better for him to have a project than to be looking for one. If you can't commit to completing a project reasonably, then say no.
And building off that, another point: if the project, for you, is more about you than someone else, then say no. No one asks you to do something for them that they don't need done. No one asks you to take time out of your schedule to do something they don't care about. So everything you do fits into someone else's story in a way that you have to be mindful of. If you lose sight of this, you'll lose sight of the real service that you're offering, and it will become lesser than it should be. So if you're prone to lose sight of others, then say no (at least until you can focus on their needs and fit it into their story). Sorry, but it's their thing, not yours.
There are a lot of reasons to say no. Good reasons. Holy reasons. If the result of your saying yes is not going to bring glory to God, then no matter how skilled you are, no matter how much free time you have, no matter how much you love the person who is asking, say no. It's as simple as that.
And if you find yourself saying no a lot, if you're struggling to find something to say yes to, then maybe it's time to examine your own heart. What's keeping you from committing to things? What's keeping you from being able to offer your best? What's keeping you from the excellence that God has called you to? If you can never seem to offer these things, it's time to ask yourself why.