While the above post(s) ARE indeed very informative and quite correct, and so is John's blog post, I find that there are many aspects missing.
The main part about getting published is NOT to get your works on a piece of paper, fold it, staple it and send it... somewhere. It's not even about selling it, or collecting royalties, or anything like that. It's about exposure!
And it's already quite difficult to present yourself in a great, professional way, if you're self published, let alone gather all the exposure you deserve. The fact that you're self published, also means that you've never been 'judged', you've never put yourself out for exposure, in order to publish your works. I bet my son (aged 7) can publish his scores (should he write music and this IS too much, I know).
The promotion you will get when you enter a publishing house, should be big.
If you put down the amount of work it takes to market yourself, to present you in a decent, professional manner, etc, you might realise that things are not that easy after all! Not impossible but still.
And just to make myself clear: Self publishing IS great and IS possible. But if one is to do it, they better do all the research and do a GREAT job, which involves much more than just a printed and a copy of Sibelius/Finale... ;) (and an e-mail to ASCAP, ok...)