On a greater camera like the D700 and above (D300 included), you get a dedicated AF-ON button at the back of the camera body. That helps me confirm today that the Nikon D90 can also configure the AE-L/AF-L button to perform similar role.
But what's the point exactly of using the AF-ON button when half-depressing the shutter release does the same job?
Here's the best answer I hope would be useful to you... adapted from a D300 user.
"From a wise man on another forum; this is now how I have my D300 set up:
- Change your menu such that AF is activated ONLY with the AF-ON button, and
- Change to AF-C release priority.
- Put your focus mode switch in AF-C and leave it there.
- Manually focus: just don't touch the AF-ON button
- Focus and recompose: put the focus point on subject, push AF-ON and release. Recompose and press shutter release.
- Continuously focus: Track target with focus point while holding down AF-ON. Fire at will."