I saw you've had this up for almost a day with only one reply, and while I don't have experience in submitting magazine proposals, I do have journalism experience and a desire to help you answer your question!

I found a couple of great sites that seem to outline the process in a clear and easy to follow way, especially this one: http://www.thecraftywriter.com/2007/11/18/non-fiction-how-to-write-a-proposal/#magazine-proposal.

I'm going to walk you through the process she outlines in a shorter approach, so you can see if it's what you are looking for and go to the site if you need more details.

First, research the market to see which magazines might gel best with your idea.

For your first contact with the magazine, it is best to start with a formal email proposal. (She also outlines the more informal phone pitch.)

This email should include:
1. A cover letter - the cover letter should briefly summarize your article idea, tell why it fits this magazine, who you are (what cred you have as a writer) and the article's word count and image availability (see her example letter here: http://www.thecraftywriter.com/files/200711/covering-letter.pdf)

2. An attachment that summarizes your article - This needs to include, and I am directly quoting here:
* A suggested title
* A suggested intro paragraph
* The main points of your article in bullet form
* A suggested conclusion
* Possible sidebars / text boxes
* Photographic possibilities
* Projected word count
* Your contact details (these should appear on the letter and outline as the two may become detached).
(See her example outline here: http://www.thecraftywriter.com/files/200711/article-summary-proposal.pdf)

Good luck, and I hope this is helpful!

Interesting I am currently learning to submit photographs to magazines, and in this process I have also come across the procedures to submit articles. As it turns out there are a great many magazines that will accept articles, however they will not approve a proposal, you have to actually submit a finished written article. Many of them ask that you include professional photographs to accompany your article. the best resource in the world is called the Writers Market (I am using a similar book called the photographers market) The Writers Market will actually list the specific guidlines for submission of articles to particular publications. You can go to Borders or Barnes and noble and just flip through the book, they list tons of magazines and other places that writers can get published and tell you how much each magazine pays, some actually pay really well, while others don't pay at all. If you have a specific magazine in mind you can look on their website, they usually list the submisson requirements. The thing that I am learning with photography and I'm sure it applys to writing is that you absolutley must follow the quidlines verbatum, you can get immediatly rejected just because you didn't submit it in the format that they like. Good luck!