as a general suggestion: in addition to sending samples to art directors/departments, send it to editors. editors get far fewer samples (ADs get hundreds a week) and are more likely to look at them. also be sure to always send things to a specific person ("tom watson") and NOT just a title ("the editor").
also, it really helps to take an hour of your time, go to the bookstore, find books that look like what you do/want to do, then find out the publisher, art director, and editor (sometimes ADs and editors are listed on the book's copyright page, otherwise you can google them in 2 seconds). while it's nice to promote as widely as possible, you don't want to waste time sending a pile of violent dragon pics to someone/somewhere that tends to do goofy little ballerina books. and also, if you're calling or e-mailing, you can say "i liked your book blah blah blah" which never hurts.
whenever possible (assuming you live semi-near NYC), you should try to drop off your portfolio, or even have arrange a sit-down meeting. if you drop off a physical portfolio (usually on a particular drop day, call ahead to find when), people will more likely make time to look at it, as opposed to art in an email, or a card.
in addition to being a semi-unsuccessful ch book artist, i used to work at a publisher (one of penguin's many ch book imprints) so i've seen it from the inside. and i can tell you that, as the assistant to the art department (or whatever the title was), i was the absolute lowest person in not only my department, but possibly the entire company. and yet I was the person in charge of reviewing portfolios etc, and then forwarding possibly passable work to the AD, who would peruse it in her rare spare time. oh lord the horror stories i could tell you about bad submissions!
there's lots more, but that's the basics. in theory it's shockingly easy to do the research and get your work seen by the people who need to see it ? especially thanks to the internet!
another resource is http://www.cbcbooks.org/
the children's book council, which is funded by publishers and includes all the basic contact information. but again, one of the best things you can do is take a few extra research minutes to find out specifically who you want to contact.
the portfolio stuff is a separate issue...
hope it helps!