You might want to consider the Sawyer gravity filters. These are claimed to be effective for filtering over a million gallons. To clean them you just occasionally feed pressurized water through it backwards. Only downside is that I think they drop that claim if you let it get outside of a certain temperature range (like below freezing).
http://www.moontrail.com/sawyer-point-zero-two-purifier.phphttp://www.moontrail.com/sawyer-sp194.php 
