You can occasionally get a contract by coming up with a killer idea, phoning a publisher, pitching the idea over the phone, and hitting the jackpot… but you’ll stand a much better chance of getting the contract you want with the right publisher by making a planned presentation.
Most publishers let you submit via email or their website, but it may happen that you occasionally submit a proposal via snail mail. This is most likely to happen if you’re including samples of one kind or another that need to be printed. If so, don’t make the publisher return copies of anything to you! Publishers are up to their collective eyebrows in submitted material of various kinds (this bunch of stuff is known affectionately in the industry as “the slushpile”) and they don’t need the hassle of returning anything. Assume that anything you send to them won’t be returned; marking printed copies in red ink on the front page with “May be discarded after review” makes their lives even easier.
Remember that you’re selling your idea and your abilities as an author to the publisher, so it’s important for your proposal to shine. Publishers respond best to an idea if they can see that you’re excited about it, there’s a marketing niche, and that you know what to do to bring the book to fruition. If you think of your complete book proposal as a job interview by mail, you won’t be far wrong. Make sure the proposal is dressed well and looks good when it first meets the publisher.