As much as I hate to admit it, Nancy Reagan was right. Sometimes "Just say no" really is good advice.
Usually, saying yes is a good thing. Yes is an enabling word. It's a move-forward kind of word. It's a get things done kind of word. I recently listened to the audio version of Tina Fey's excellent Bossypants, and I loved what she had to say about improv theater. Here's her first rule of improv (summarized by me):
1) Agree. Always agree and say yes. You are required to agree with whatever your partner has created. The rule of agreement reminds you to respect what your partner has created. Start from a yes, and see where that takes you.
That's a great rule. It's particularly great for creative work, when an early "no" can be crippling. Say yes, and see where that takes you.
Where saying yes all the time gets you into trouble is with obligations. Yes, I'll come to your book festival! Yes, I'll speak at your conference! Yes, I'll judge your writing contest! Yes, I'll critique your manuscript! Yes, I'll blurb your book! Yes, I'll do your interview! Yes, I'll write an essay for your anthology!
Oh, and yes, I suppose I'll get that book written too.
Don't get me wrong--I want to do all those things. All those things are a big part of why I wanted to be an author in the first place. Before I was published, I wanted to be a guest at book festivals and conferences, I wanted to be asked to judge writing contests and critique manuscripts and blurb books. I wanted to give interviews and write for anthologies.
And I still do. But I've learned I have to start saying no to some things. I just don't have time to do all the writing things I want to do that aren't actually, you know, writing. I've had to walk away from some of those things. I've had to prioritize.
The only person who is really going to guard your writing time is you. It's hard, but saying no is sometimes the most important thing you can do.