The later /5 and /6 models had a "steering dampener" knob which basically put more reload on the bearings, but that was phased out during the /7 models I believe - at least it was gone by the 1981 model year. You can adjust the preload as Justin indicated but just be careful about it - too much preload can not only wear out your bearings fast but can also make the bike behave/steer VERY differently.
BMW intended the adjustable knob on the earlier airheads to be cranked up only when riding on very bumpy roads, though many folks cranked it in all the time.