AM has a great powerpoint about omnidirectional drive, which has several diagrams that may make seeing the vectors talked about above easier to see.
http://www.andymark.biz/presentations.html
Also, if you use a gamepad or similar control device, using the left joystick to control the movement of the actual robot and using the right joystick x axis to rotate the robot, it makes it very easy for someone who has played Halo to figure out how to drive your robot.