The two foregoing posts are spot on, all of the mechanism of the nose strut must be a very close fit - zero sloppy movement. Ive rebuilt many of these struts. If there are any loose points in the mechanism, a gentle hand ferry flight for maintenance only. If there is a nosewheel fairing, remove it, they worsen shimmy. Inflate the nose tire to the highest allowable pressure, and have it balanced when possible.
If the aircraft has any wing mods (STOL kit, particularly Robertson), or Flint Tips, these make the elevator even less effective at raising the nose.
If you're training someone on a 206, insist that the nosewheel be held light for all ground operations - full nose up elevator during any rolling slower than flying speeds. No tolerance for letting the nose be heavy. Certainly no tolerance for anything like a nosewheel fist landing! If the candidate pilot is having trouble, have them land with no more than 20 flap until they get used to it. All taxiing should be done with 15 degrees of flap extended, and full up elevator control. The changed downwash over the tail will assist in keeping the loads off the nosewheel. 206's are particularly vulnerable to very expensive nose structure damage from mis handling.