Best description I could have hoped to think of.
After a lap or two I have a good idea what gear I need to be in, or if I need to downshift at all, through most corners on a track. I count downshifts in my head sometimes, but it doesn't take too long before you develop a good sense about when to stop downshifting. If you time it correctly, doing what Duke described results, most of the time, in you being in the correct gear when you stop braking and go for the accelerator (less of the time however with really slow cars that stop on a dime, need to be careful not to go too far with those, if you've gotten used to faster cars).