Hi Neil,
I think the best approach is the direct approach. Most magazines have a picture editor, or features editor and you will have more chance of selling your images if you can also offer a few paragraphs of text by way of an article as well.
Remember, magazines work several months ahead of themselves, so winter stuff is probably now pouring in to their offices and already chosen. Also, a lot magazines have regular contributors who are invited to supply stuff via commission and so do not accept spec work from outsiders. A good way of finding the best mags etc to approach is to get the latest copy of the Freelance Photographers Handbook:
http://www.amazon.co.uk/Freelance-Ph.../dp/0907297617
It will give you all you need to know about markets and what they buy. You could also send regular work in to photography magazines, but be prepared to wait a long time for a response (my experience anyway). Another sure fire method of getting published is to specialise in a particular area and become the best you can at providing images for it. Remember, you will also be up against images supplied by libraries as well.
If you write to a magazines editor and ask for submission details they will usually have a form letter outlining their current needs and T&C. Also, if you have stuff you want to send to them, and you haven't first been invited to do so, write to the editor, briefly introduce yourself and ask if it would be fine to send in a sample of what you have. Keep the letter short and to the point, or they will just file it in the bin for the cleaner to read.
I used to belong to the BFP a few years ago. You get a copy of the handbook I mentioned above and a regular market news letter that points out what the various companies are looking for. The cost is no dearer than a magzine subscription to join and is well worth the money. Here's the link for the BFP
http://www.thebfp.com/index.php?page=2
Magazine staff are generally manically busy people if their magazine is a big seller, so have patience with them and good luck.