There is a lot of information 'out there' for writers, including this here website. Lots of dos, lots of don'ts, lots of advice in general. Still, though - STILL! - the same errors crop up in submissions over and over again. So here's a short list - a reminder, if you will - of what not to do.
1. Do not send out your manuscript if it's still at first-draft stage - there will always be room to improve after that, and you need to send out the best possible version of your work.
2. Do not send out your manuscript if you know, in your gut (or your heart - whichever you prefer), that it isn't ready, even if it's had several drafts. You only have one chance to submit to agents and publishers - once you've been rejected, it is highly unlikely that the same manuscript will be looked at again. And, deep down, you know when it's still not ready - you're just trying to talk yourself out of it.
3. Do not send your submission to someone who isn't interested in the genre or category of book you're writing. Children's authors, you're the big culprits here - many of you send submissions to agents and publishers who don't represent or publish children's books.
4. Do not ignore the submission guidelines - they're arbitrary, yes, but they're our attempt to create order out of chaos.
That's my short Friday afternoon list. If you want some more pointers, play this game.