And one more angle on understanding dish: Forget the terminology. Just make sure the rear rim is centered between the brake shoes. If you've got a disc brake, just make sure the rear rim is centered between the seat stays. You should be able to put the rear wheel between the dropouts with the cassette facing either side, and the rim won't shift position between the brake shoes/seat stays. Then, your wheel is "dished" correctly, and you'll notice that the angles the left and right spokes form with the hub are different.
Continue on to steps 2-4 listed above. :-)