If you want to use multiple shields, you need to use stacking headers like these:
Shield stacking headers for Arduino (R3 Compatible) : ID 85 : $1.95 : Adafruit Industries, Unique & fun DIY electronics and kits.
If you purchase the motor sheild from adafruit, I believe they even have to option of it coming as a kit which means you don't solder in the normal pins and instead, solder in the stacking header pins. Then you put the sensor expansion shield on top of your motor sheild.