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Focus often changes with temperature, too - making things tricky. I find wide aperture primes easier to focus than zooms since they're usually so much brighter for LiveView (don't forget to turn on the ISO expansion custom function if your SLR has it), and the optical aberrations you get with them wide open actually help focusing.
Another trick I use with my 20D (doesn't have LiveView) is to put a piece of tape on the focus ring of the lens when it's at "infinity", then take a series of long test exposures, rotating the focus ring by a notch between exposures. Zooming into the photos will quickly show where the proper focus lies (i.e. between two notches).
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