What works well for me is a product like Goo Gone. Just a little bit on a rag wiped along the fretboard will get the job done. If there is lots of gunk on there, a few carefull swipes with a scotchbrite pad will clean things out in a hurry. It doesn't need to be used everytime you change the strings, perhaps every 3rd change or so. The stuff is citrus based, and won't wreck your hands. I know that using it is okay on newer polyurethane finishes, but I'm not sure at all about nitro. As has been mentioned before, I also put some Fast Fret on after cleaning.