Foreigngirl said it best, if you want it to have the greatest effect, say it at times when it's not expected. Saying it then says that you're thinking of them at that given moment of non-significant time and can show that you may be thinking of them at many other times through the day, which if you're truly sincere, you probably are. Saying it when you leave off somewhere or stop talking to someone seems so much like a formality. Like it's the closing line to make sure that the people know each one loves the other.