If you self-publish your book, you know you're signing up for a ton of marketing. There is no company behind you, no financial backing, no support group (other than your friends and family). On the other hand, everyone seems to think that if one can just "get an agent," very little of that stays the same.
The truth of the matter is, even if you go with a traditional publisher, you better be prepared to do some heavy marketing. Some of the best ideas I have ever seen to increase sales and your own public recognition have come from self-publishing blogs. A great one is by James Betcher. If you are a contracted author, don't be so proud (or foolish) to think you don't need these tips and information.
I want to mention one tip Betcher writes about that I found very, very useful. He said that his "most successful marketing strategy has been identifying Amazon Top Reviewers and emailing them, asking if they'd consider reviewing my book." On his blog, Betcher gives some advice for how he approached the Top Reviewers, so be sure to check out that link attached to his name. But it paid off. To have one of them give you a glowing review bumps your book closer to the top of the stack on Amazon.com. Which, let's face it, it's bigger than Walmart, at least online.
I am always surprised when I see or hear of authors publicly complaining that their publisher is not doing the marketing they expect. I read a blog yesterday where an author that I am familiar with did this. I know this author well enough to know that he is doing ZIP to promote his book. I've done more on my own blog for him, publicizing, book reviews, etc. If the author is not willing to put forth some effort, what makes them think that the publisher is going to have the confidence to do so? It's a joint effort. (To read more about this author and the ongoing debate he sparked, check out the blog here.)
So really, what's the difference between self-publishing and traditional publishing when it comes to marketing? There shouldn't be any. You should promote yourself and your book as aggressively as any self-published author.