Electrically Controlling A motor direction
You want to use an H-bridge, which is nothing more than four electronic switches hooked up in a nifty way that lets you change the direction of current through a motor. THIS is a pretty nice review. 


From http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/

Don't try to build one- buy one like the SN754410.  You need to be a little careful if you see "half bridge". Two half bridges are used to make a full H-bridge.  That chip has four half bridges.