This requires not that the writer make all sentences short or avoid all detail and treat subjects only in outline, but that every word count.
(Strunk & White)
The above from Strunk & White is a constant rule that I revisit. After writing a chapter that I think is the best ever, when I edit, 2/3 is telling not showing, and I've written unnecessary words. I admit it. I'm a chatter box. My brain jumps around like a frog trying out all the lily pads on his pond. I won't tell you how many words this post had before I kicked out the unnecessary.
I'm working hard on pruning-out the deadwood in my YA. Cutting those precious little darlings is painful. But, I must. And find those showing words that fuel my story. Unnecessary words are nothing more than reading clutter. Remember, less is better, always.