I use the command "target target's target" on one of my mouse buttons.

When the tank does a pull, I click it and it immediately targets whichever mob the tank targeted.
If I think the tank switched targets, I click it twice. Once to retarget the tank, and again to target the tank's target.

It's also really useful as a healer, because I can quickly switch DPS (tank's target) and heal (the mob's target. In other words, the tank).