I was recently asked “What is the process for pitching an idea to a publisher?”

Although I am far from an expert, I am going through the process now of submitting my manuscript to publishers and agents, and so this was my reply:

It depends on the type of book. For fiction, most publishers want a finished manuscript, not an idea. For business books and the like, some publishers will consider something in the idea phase, but usually only from established authors.
Having an agent is a bonus, not a prerequisite (as some other people have suggested). If you have a finished manuscript you can be submitting query letters to agents at the same time that you are submitting to publishers.
Duotrope Digest (http://www.duotrope.com/) is an excellent resource to find publishers. You pick the genre of your book, the length, and you get back a list of publishers who are open to submissions. In addition, I highly recommend Preditors & Editors (http://pred-ed.com/), another website that allows you to review publishers, and get recommendations on which ones are good, and which are to be avoided (because of bad contracts, conflicts of interest, negligent on payments, etc.)
When you do have a genre, length, and finished manuscript, by using the two resources mentioned above, you should be able to make a list of 10 to 15 publishers that are a good match for your work. (Perhaps less if you have a novella length work, for which there are few publishers.) Start with the most reputable and well known publisher, and work your way down the list. You don’t want to sell yourself short by starting at the bottom of the list.
The best known and most reputable publishers will have the widest distribution, best sales and editors, and will do the best job of getting your book out there. However, just getting a first book published by any publisher (not a vanity press or self-publisher) will enhance the opportunity to get future books published, so it is perfectly reasonable to go with a smaller publisher too, if you are not accepted by the biggest/best.
It’s helpful to have a blog or twitter, and establish a following, because that feeds into the publishers consideration: if you will be able to help promote the book, then you will increase sales. On the other hand, publishers don’t like if you self-publish. (The merits of self-publishing is another whole topic – but if you want to be published by a regular publisher, they are biased against people who self-publish.)
Perseverence is key. Many published authors submitted their manuscripts dozens of times, likely spanning several years, before having it accepted. Authors talk about keeping the pipeline full: one you have one work making the rounds of publishers, get started on the second work, and then get the second making the rounds. Then the third, and so on.