Lonely Planet's Guide to Travel Writing is a great resource for aspiring travel writers. They profile a bunch of seasoned, professional travel writers and give some practical advice about how to pitch and place travel articles in markets like the US, UK and Australia.
Like any books on writing, the main theme is to practice! Write. Lots! It's the only way to refine your craft.
And in the mean time, I shall keep blogging about travel and waiting for my dream travel writing/documentary production job to come flying through the door. *sigh*....must be time to book another trip!