This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Occasionally you may find the pedals suddenly more difficult to
disengage. This usually happens because dirt or other contaminants
get caught in the cleat or pedal mechanism. I have found that a good
spray with a hose quickly and cleanly washes off dust, mud, or other
gunk from the pedal and cleat. You may also wish to spray the pedal
with a light silicone or teflon lubricant.
John Unruh (email@example.com)
Lawrence You (firstname.lastname@example.org)