If its a strong trend,like the Dow, any weakness such as a lower close or a lower high: alternatively, second (or occasionally third) consecutive higher close.
If in a trend but not such a strong feature, sometimes the reversal of slope of the 20EMA. Sometimes, in forex, an increased weakness of the trend across the major pairs of the same currency.
If the trend is strong I don't mind getting out early because the exit signal is often also a new entry opportunity: I really don't mind getting out this evening and entering on a new entry order tomorrow morning. On the other hand, if the trend is weak, why would I worry about getting out early?