It ain't that bad. Now with the latest patch you can be lazy and use only one button for your pally and do pretty well. Something like...

/castsequence Hammer of the Righteous, Consecration, Holy Shield

... should do wonders. Surely you can find one button to free up on the shammies to have nothing on it so it can be your "pally only attack" button.

Alternatively, with only a bit more effort, put your main shammy attack button (lightning bolt, most likely) on the same key as the pally attack button and simply click with your mouse on the pally button the first couple of times you face a new group (so only the pally attacks) if you are worried about agro, then spam the button (which becomes your "everyone attacks" button) to have everyone attack.

If you get really advanced you can use your mouse to click on a seal and judgement on your pally while spamming the "everyone attacks" button, but it's almost never needed.