Originally Posted by greenstork
The only time I've ever really had a problem with this is when I've tried to really pull the stem through the hole in the rim to set the nut. Now... when I change out tubes and tires, I leave the nut just barely threaded when setting the tire on the rim and pumping it up. Once pumped, it's usually sticking out to a normal level and then I thread the nut all the way down, being careful to not really pull on it.
This advice is, of course, applicable only for Presta valves with threaded valve stems.
First thing I do with a new tube is throw the cap and nut in the trash.