A simple tip that may help to get sharper photos (applicable for at least some Nikon cameras)

I was looking for a while for the technique that would make my photos sharper. Since I have switched my gear to Nikon D800E I have noticed that in some situations the sharpness of my pictures was not satisfactory. Initially it could be explained (in some cases) by left focus point issue, but after readjusting of the autofocus system by the Nikon service, I was still sometimes struggling with getting sharp pictures, especially in the AF-C autofocus mode.
I started analysing the possible reasons of the problem. One of the things that came to my mind was the technique that I use quite often, namely locking of the autofocus and recomposing the picture. In the AF-S mode the behavior of camera is simple: it focuses, lock the focus while the shutter release button is pressed half-way and then one can recompose the picture by moving the camera.
In AF-C the situation is slightly different. The D800E manual states on p. 91 that in AF-C mode camera focuses continuously while shutter release button is pressed half way. So simple half-press shutter release and recompose technique will not work (when one moves the camera to recompose, the gear will adjust the focus to a new situation).
To lock the focus in this mode one needs to press and hold the AE-L/AF-L button. I have tried it but found not very comfortable and convenient.
It is more convenient method to lock the focus in the AF-C (and in AF-S) mode. Use the AF-ON button to lock the focus instead of pressing the shutter release button half way.
How does it work? Quite simple, actually. To focus, press the AF-ON button. Camera and the lens will set up the focus to get the image sharp. If you release the AF-ON button, the focus will remain locked on the position where the focus was gained. So using this button decouples the autofocus action from the shutter release button. While for the AF-S mode it does not matter so much, for AF-C it allows to lock the focus just by using single button (AF-ON), instead AE-L/AF-L and shutter release.
When I started using this technique I was a bit skeptical since I was worried about learning a new habit to lock the focus by pressing something different than shutter release button. But it turned to be an easy and quick adaptation and now I get completely used to use it.
How does it help to get the sharper pictures? First of all, while recomposing in the AF-C mode the sequence of buttons to press is simpler: AF-ON press, AF-ON release, (recompose), shutter release.
Second (this may be my subjective feeling), I think that by using a thumb to press AF-ON button, I enforce more stable camera holding. So even if I track the objects photographed in the AF-C mode to have them constantly in focus, by using the AF-ON button I embrace the camera with my right hand in such way that it remains more steady.

To set up the AF-ON button for locking the focus, some settings need  to be adjusted.

Technicalities - how to setup AF-ON to lock the focus.

Initially the D800E is set in such a way that the AF-ON button locks the focus exactly the same as pressing the shutter release button half-way. To achieve the behavior described above one need to block the focus lock by sutter release button.
To do so set the Autofocus->AF activation option (a4) to AF-ON only (OFF) in the Custom Setting Menu.
In my menu navigation notation:

MENU, , ,,, 3x, ,, , AF-ON Only, Ok

 If you have the Nikon MB-D12 battery grip and want to setup the AF-ON button to lock focus, use the option Controls->Assign MB-D12 AF-ON option (f13) to AF-ON in the Custom Setting Menu:
MENU, , ,,, 5x , , AF-ON, Ok

Don't forget to set those settings for all custom menus where you plan to use them.

The updated version of the D800 settings cheat sheet can be found here.

Note that AF-ON button is available on the other Nikon cameras as well so this technique does not apply only for D800. Check your manual for the details on how to set it up in case of your camera.