Remember, this is Nikon country; I use the D300 specifically, note that the F100 can be setup in the same manner
– you might (should!) be able to do the same with other cameras too.
I have the AF assist light off; I find the light to be of more hindrance than help – often alerting the subject about me trying to take photos.
I think IR-based AF help is another matter, not having this problem – likely available on e.g. Nikon flashes SB-600 … SB-900.
Pushing the the multiselector (D300 English manual: p7, item 6) should activate the center AF point
(Description: p300, about custom setting f1, set to default).
In low light situations I turn on spot metering (by item 10, p7) and use the center af point positioned on something that has contrast (e.g. dark to light transition) at the range where I want focus to be.
As always you can choose where the “normal spot” (18% grey!) is to be in the photo – the photo YOU wish to take;
place the focus point over it, push AE-lock, then go on shooting like before. AE-lock again to regain normal function.
Now all there is left to tell; make sure just AF-ON (item 4, p7) activates autofocus (related D300 custom setting is a5), that is; DO remove AF activation from the shutter button, this allows to take several photos without having to refocus. Push the AF-ON button to select focusing distance (focus point over a “good spot”) and then compose and wait for the right moment for shooting – with focus prepared.
This way you have decent control over what AF does.