Always wait at the gate if the numbers show availability. Of course, that doesn't ensure an upgrade, but the odds are way better in your favor than if you board early. Yet oddities do happen too. I waited at the Gate when flying on LH out of Munich to Cape Town. I was #1 on the upgrade waitlist. Business Class was showing full, however, one passenger, who was on another connecting flight had not checked in due to that flight being delayed. Just as the GA was about to process the upgrade, that passenger came running up to the Gate breathless. It was a long flight to SA in coach.