On changing key, if you dislike having natural signs negating the sharps or flats of a previous key, there is a workaround.
At the end of the previous bar (measure) insert the new key. This will have the negating natural signs. Make this invisible. (select it, and then hit ALT and ENTER. Choose the visibility tab and follow your nose). Insert the new key again after the bar line. This will not have any negating natural signs.
A corollary to Richard's suggestion: I remember seeing scores in which the negating naturals were placed before a double barline, with the new key signature after the double bar line.
You can implement this effect by placing a key change to the key of C before the barline, and the desired new key after it.