Switching your camera’s focus mode to AF-C (auto-focus continuous mode) largely eliminates this problem. AF-C mode automatically compensates for any “body sway,” or any other subject-distance variance from the intended focus-plane, subsequent to your initial focus acquisition. Also, using the AF-ON button (which activates auto-focus, independently of shutter-release) is really just a matter of preference, but I find I focus more accurately using AF-ON in tandem with AF-C mode.
I used to shoot in AF-S mode, using the the center-AF point to acquire focus, then re-composing my frame to accommodate my desired composition (commonly referred to as “focus-recompose”). But I found that the recompose often induced noticeable focus error when shooting fast lenses wide-open. During that slight framing adjustment, the subject-to-image plane distance can change enough to throw your focus-plane either too far forward, or too far rearward of where intended, especially when your depth-of-field is razor-thin.
Now, I shoot in AF-C mode 95% of the time. I use the AF-ON button, single-point focus-area mode (where I select the active AF-point), and AF-C mode as my standard AF set-up. Also, make sure that any “dynamic” focusing modes are turned off in your camera’s AF menu. If focus is really critical, I’ll hover an active AF-point over my subject’s eye to be sure to nail it. Shooting this way, 95% of my wide-open images are now in-focus.
Note that the new Nikon SB-910 will illuminate its AF-assist light using any focus point (not just the center point) in AF-S mode (but, not in AF-C mode).