I'm mainly an illustrator, but I'm working on writing picture books too.
Lin Oliver quoted Richard Peck as saying, "For every one book you write, read 1000." I know I've read loads of picture books these past couple years, but I don't know if I've hit 1000 - possibly. I always have a huge stack of picture books checked out from the library. A couple months ago I gave myself a Picture Book Reading Challenge. I checked out over 60 picture books from a great-read-out-loud list and read them all that one night! I felt like an editor reading slush (except these were published books, so probably a whole lot better than most slush).
I divided them into three piles: 1) books I loved and wanted to read again, 2) books I thought were ok but didn't want to read again, and 3) books I thought were boring or not to my liking. I ended up with only 6 books in the "books I loved and wanted to read again" pile.
Then I read my picture book manuscripts I'm writing. Here's the real meaty question: Which pile do my work-in-progress manuscripts belong? All of them went in the middle "ok" pile.
How do I get my manuscripts into the best pile so that they'll actually be acquired? I studied in-depth each of the books I put in the best pile to see what kind of books I love and what I loved about them. I found out that I love picture books that are hilarious, clever, fast reads, and that the illustrations tell at least half the story, sometimes even contradicting the words to be funny. I decided to rewrite one of my manuscripts completely with the intent of it landing in my "books I loved and wanted to read again" pile. After rewriting it (and drawing as I wrote), I feel it's much stronger, and now I'm sketching it into a dummy to submit to publishers. (If you're an illustrator as well as author, you can submit a sketch dummy instead of just the manuscript to publishers.)
So if any of you picture book writers or illustrators want to check out loads of picture books, read them all in one sitting, divide them into piles based on your tastes, and study the ones in the best pile, I hope it'll help you too! That's my challenge for you. And I may join you and do it again with another big stack of books.