A handy way of remembering this rule is thus: "Break the line, change the sign"
You'll have to use your imagination on this. If I have ¬(A∧B), imagine that that is AB with a line over it. That is equivalent to ¬A∨¬B, which would be A with an overline and B with a separate overline. When the line is broken, the sign is switched. So if you had ¬(¬C∨D), the two overlines attached to C would counteract each other, breaking the line, and the result would be C∧¬D.
References: my Digital Systems teacher