The method expalined here has worked great for me :
http://www.tigoe.net/pcomp/code/category/code/picbasic-pro/62
It's just at transistor and a diode and a resistor.
You should be very careful with running motors directly fom Arduno pins, all but the smallest motors can easily draw a lot more current than the 40 mA an Arduino pin can supply. And even if a motor is rated at XX mA it will draw a lot more just when starting up.