Your best option is actually word of mouth. Once someone has given you a try and they like you they will start recommending you. You can also make little flyers or post it on Facebook or something too :) (I house-sit and all of the people who contact me have been told about me by someone else)
There are websites like www.mindmyhouse.com/ that claim they can hook yiou up with people all around the country. All you have to do is pay transportation. Had a friend who would do that sort of thing often. Once had to spend two week in a house on the beach in Malibu. Main duties: Water the house plants, and take the dog for a walk along the beach each day.
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