I wouldn't connect the ground to the Rheostat. It will add a second load in parallel to it that draws about 360 mA; quite a lot from a 9v battery. In a series circuit, consider both the motor and the rheostat resistors. The higher resistance of the two will have more voltage going acrossed it, they will both have the same current. The voltages of the two (or more) will always sum up to 9V. As the resistance increases, the current drops. Obviously, with less voltage, comes less speed. If you want the lowest speed turned to the left and the highest speed turned to the right, wire it like this (shown from the back side, not the adjustment side):
As well as that, I recomend wiring in series with that, a resistor 2-5 times the value of the motor resistance (I hope you have a multimeter!) and is able to handle the wattage. This will keep the voltage and current down and keep you from causing a short with you motor and blowing up your battery and\or motor (I'm guessing it's less than 5 ohms). It's also common practice to wire a .1uF (104) cap in parallel with the motor. I highly recommend reading up on Ohm's Law if you haven't done so already. Do a search, there's plenty of sites that give you that info for free.