A new subtraction game is now ready for downloading over on my website. This game focuses on the subtraction doubles ie 7 + 7 = 14 so 14 - 7 = 7. The awareness and mastery of these facts make for more effiecent mental computations.
The students have a great deal of fun playing this game but it is very important to discuss the strategies involved with being successful in this game.
I also discuss how instant recall of number facts leads to efficient mental computations. Try getting the kids to do the full algorithm with a calculator while you or a group of kids arrive at answers mentally. The kids like seeing that the human mind is quicker than the calculator. 
I also tend to demonstrate how counting backwards on fingers is a effective strategy for arriving at an answer but that it is not efficient.
Have fun with Subtraction Doubles.
Adrian