There are a lot of helpful how-to books on travel writing (e.g. The Travel Writer's Handbook by Zobel and Butler). But my advice is to spend more time actually reading travel writers who inspire you.
Some of my current faves (which include novelists and foreign correspondents) are Pico Iyer, Jan Morris, Alison Wearing, Jill Lawless, Rigoberto González. A good anthology is AWOL: Tales for Travel-Inspired Minds, edited by Barclay and Logan.
Online resources
You can join The Travel Media Association of Canada once you're getting published regularly in some high-circulating publications. (The median age of the membership may be a clue to how hard this is to do.) Their website has an interesting Media Code of Conduct that helps me stay out of trouble.
Amanda Castleman, an award-winning travel writer has gathered an impressive list of online resources for all aspects of the field.
Writer's Weekly has information on both books and freelance writing work and includes a message board for writers and a jobs board.
Travel Blog Exchange is a community of bloggers worldwide that hosts networking conferences around the world.
Travel Writers Exchange is a both a forum and an excellent resource for articles on travel writing.
WriterFind is a site posting freelance and telecommuting jobs.