S is one of the most valuable letters in Words With Friends, which is why it’s so important to use it in a calculated manner (WWF has five S tiles and two blank tiles that you or your opponent can use as an S).
Players should use the S tile in two instances:
1) Use S to pluralize and lengthen your word in order to land on a double or triple word tile. There are so many words you can easily extend using this strategy. Just remember -- pluralizing your word is only recommended when S helps you score extra points from a bonus tile.
2) Use S to pluralize a word already on the board while simultaneously building a new word. Keep an eye out for words on your tile tray that contain S -- once you’ve spotted one, place the S at the end of a word already on the board and build your new word from there. While choosing a word on the board to pluralize, remember to always keep bonus tiles in mind.
You should only use your S tiles for plays worth 30+ points or Bingos; you’re better off saving your S’s until you have a move that meets these criteria.