Currently the strongest Scrabble-playing program in the world is Quackle, which uses simulation as mentioned by ColonelPanic. It generally only simulates a few plies into the future, which is almost always good enough for a game like Scrabble with a significant random element. Also, Quackle's board is completely configurable so it should be able to play Words With Friends as well. The code is open-source and may be instructive. I don't know if this helps but I hope so!