sprinkler head problem

My lawn has those pop-up sprinklers with flat covers that retract flat when not in use. They're the ones that look like a big, black vase on the outside.
I've had problems with the things filling up with dirt. This, of course, creates problems with the the heads either getting into fixed positions, or ending up sprinkling other than where they're supposed to.
Is there any easy fix for the problem? These things are not easy to work on in the ground, as they're spring loaded and constantly want to retract on you while you're trying to work on 'em.
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