Some of the useful tips I picked up were:
- it can take many, many revisions before a picture book is ready to send out
- keep copies of your old drafts in case you want to go back to them
- it is not just a case of writing a 500-word (or however many words) short piece of text and calling that a picture book. There's a particular craft involved in picture book writing that takes time and effort and attention to master
- you should always have someone else (and preferably more than one person) give a critique on your picture book manuscript. You need someone who can say - "Have you thought about this?" or "I'm not sure this part works"
- once you have edited your picture book manuscript and sent it out for critiques, take those comments on board and begin the re-editing process again
- some authors have revised a particular picture book manuscript 60 times before it is "finished"
In the end though, you'll know when it's ready! Trust your instinct, and remember there's nothing to say you can't come back to your picture book manuscript down the track and begin revisions again...