Using a transistor instead of a relay to control a DC motor
You can use a logic gate MOSFET, like the FDC855, and then your circuit will look like this:



A FET will give you a high current if needed (probably not necessary, since you're powering the motor from AA cells).

Or a BJT like the BC547, and then you would make it like this:



With the 1 k&Omega; base resistor you'll have minimum 100 mA current for the motor. If you need more I'd recommend the FET solution.
The diode can be a 1N4148.