I will start practising! Please stop the "to DOF or not to DOF" debate, it was never my intention to label all shallowDOF maestro's as being gimmicky!
I'm guilty myself sometimes..
Sorry for hijacking the thread! It just seemed relevant, seeing you originally asked about fast lenses.
To return to how to focus, a point no-one has mentioned is 'equivalent focus'.
In other words, although the human eye is surprsingly good at judging distance (or can be trained to be so), it is even better at judging which of two things is closer/further away, and which are at the same distance.
Now, let's say you want someone's eye in the upper corner of a pic. You can see how far away that is, and you can see something else that is at the equivalent distance, but a lot closer to the centre of the image: an ear-lobe, maybe. So focus on the ear-lobe.
This may sound impossibly complex and circuitous, but then, a lot of things do when you first think about them (or worse still, try to explain them). How many equations do you solve when you catch a ball, for example?
Another technique is 'rocking'. You focus on an eye. You know it will be a few cm further away when you recompose; so you rock forwards slightly to compensate.
Both techniques are only relevant in the extreme close up, extreme aperture range.
Cheers,
R.