Once you change your position to Short, if nifty comes back to the same value(where you chaged to short), do NOT do anything, now you should be checking for the 'Stop Loss' mentioned by Vinod, Today's example should answer your question. As today this method recommended to change to 'Short' arround 4500, next after going down it came up to 4560 but still you will remain Short untill the stop loss mentioned breaks(I do not know the stop loss for today since I do not follow this method).
Hope you understood the logic.