Here's an interesting site I came across yesterday while web-surfing, following "the lynx trail" through cyberspace. When I first got a computer, I would do this for hours at a time, amazed at all the info that was available. This is another informative site, this time about watercolors. And Painterskeys is an informative newsletter for painters sent out twice a week by Robert Genn (the replies are often as good as the article itself).
Of course, just doing it is the main thing when it comes to painting outdoors, but sometimes a little information can prime the pump to get you started. Like skiing, horseback riding and making pottery, it's at least 90% practice, but it helps if you know what to practice.