So much great advice in this thread!
As some above have pointed out, reading many picture books is helpful. Especially current picture books rather than older books; I've noticed older books tend to be more wordy.
I've heard different advice re: including art notes. Some editors don't mind art notes (if they are necessary) from writers. Others would rather have NO art notes.
I've illustrated one book for Simon & Schuster Books For Young Readers (I'M BORED, written by Michael Ian Black) and am writing & illustrating a book for them. Also have a contract to illustrate another, though the project has not yet been chosen.
What I loved about the first project, which was my very first illustration project -- that I was given quite a bit of freedom in choosing how to illustrate the author's text.
I was surprised how much freedom, in fact -- up to then, I was mainly a writer, and had assumed that if I ever wrote a picture book mss that I would need to put in lots of art notes.
BUT as an illustrator, I appreciated NOT having lots of art notes -- it made the project more of a collaborative effort rather than me just illustrating to spec. I think there's a tendency of writers (I speak from experience since I used to think like this, too) to believe they need to maintain control over exactly how their picture book text is portrayed in illustration, else their story is going to get changed in bad ways.
Now, however, I have a much greater appreciation of the joy in illustrating a project in which there is implicit trust in the illustrator's abilities and creative input, and a true collaborative effort. I look at the final version of I'M BORED (I've seen the proofs; the final book comes out in September) and I see the joint effort of a creative team rather than just the author: author, illustrator, editor and art director. If anyone's curious, I'm posting about the whole process at
http://debbieohi.com/boredbook.
Having said that, however, I'm sure different publishers and editors and art directors work differently.
Thanks for this fascinating thread! I'm learning lots. :-)
Debbie