I second Anna's counting technique. We went through this too and I had posted a message and then read through one of the other threads. One mom's (momtonb&ab) tip was to count things, so I tried it. It was the only thing that worked, and now we do singing or other things as well, but mostly we count things.
We count 10 'whatevers' on the top and 10 on the bottom. I let DD pick; tonight it was 10 'muffins' on top and 10 'cousins' on the bottom. The first few times, I gave her a couple choices of what to count and she looked at me with no clue what I was talking about so I just chose something she liked (dogs), and after a couple days it stuck. Now, I ask her what she wants to count, she blurts something out, and opens her mouth. No problems!
I am a bit softer when she's teething (currently getting canines), but I don't take breaks otherwise it would be harder to get her to start again. If it's really painful, I agree with Brenda, and I just let her do it herself for a couple days.