I've had to remind myself of this advice recently as I contemplate the direction of my third novel. A great idea pops into my head, but I think, "No, I'll wait 'til the next book for that one" or "I can't write that now---what if I can write it better in a couple years?"
Sure, if I'm doing things right, I'm always going to be writing better with each year that passes (I hope!). Maybe I will be able to tell the story better down the road. Or not. That's the issue. I don't want to hold onto my best ideas, because more good ideas will come later!
An example. In my first novel, Thicker Than Blood, I have a character who doesn't even show up in any scenes but was someone who got mentioned in the thoughts of another character. Her name's Abby. Now I like the name Abby. Enough so that I didn't want to "use it up" for such a minor character. I tried changing it (she was going to be Nora, another name I like). It was a no go. This character had become Abby in my head. I felt like I wasted a good name! But as it turns out, as I wrote my second novel I had the wild idea to include this Abby character. And she took over the book! Which goes to show me the advice I heard was right. Don't hold back good ideas, or character names, for later.
Every book I write needs to be the very best it can be... right now. And this is good advice for life, too. Didn't Mark Twain say, "Don't put off for tomorrow what you can do today?" Which goes hand-in-hand with Jesus' words about not worrying about tomorrow, for tomorrow has enough troubles of its own.
Let's live life to its fullest today, no matter our vocation!