Pasting a crescent moon into a twilight or night sky adds a wonderful touch to a photograph. Many photographers use the full moon, but to me this is too cliche. The crescent moon seems more intriguing and artistic. In order to do that, it's a simple matter to make a rough selection around the moon, including some of the sky, with the lasso tool. Then copy this to the clipboard (Edit > copy), and then paste it into the night sky with Edit > paste. Next, use the 'darken' blend mode in the layers palette to blend the moon with the new background and eliminate the small amount of original sky that came through from the clipboard.
This is the huge infinity pool on the 57th floor of the Marina Bay Sands Hotel in Singapore. My settings were 15 seconds, f/8, 200 ISO, 14mm ultra wide angle lens. The water in the immediate foreground is blurred due to the movement of water in the pool.