Getting the suspension set up properly is key.
Headshake is usually caused by too steep of a steering head angle, causing the bike to be unstable. There are many variables that can cause head shake.
- Fork springs too soft (too much sag)
- Fork compression too soft/rebound too stiff (rides low when hitting bumps)
- Shock spring too stiff
- Too much preload on the shock (not enough sag)
- So basically any overall suspension imbalance that causes the rear end to ride higher than the front.
- Front tire pressure and condition of the tire also comes into play. Low pressure or a beat down sidewall will cause lots of flex and also head shake.
- Headset bearings, loose or worn, can cause head shake.
- Loose front wheel bearings can also add to the problem.
- Riding style can also add to head shake, like getting way up front on the bike in choppy high speed stuff
My personal experience is to get your weight back and hammer the gas when head shake starts. This loads the rear end and raises the front, pulling the head tube angle back into a more stable geometry.
I had a 1994 YZ125 that only suffered head shake when getting on the brakes with my weight towards the front of the bike. No amount of changes would cure it, so I just had to adapt my riding style. It wasn't a common occurance, and only happened on high speed trails.
I also had a 1999 KX125 that had HORRIBLE head shake on the first ride out. Turns out the previous owner had cranked the preload on the shock. With my 180+ lb butt on the bike it had under 2" of sag. I should have checked before ever heading to the trail. Setting the sag ALMOST completely cured the problem. Getting rid of the dry rotted 8 year old original tire did the trick.
The point I am trying to make here is that you should make sure your bike is in good working order and setup properly before throwing money at a steering damper to mask some other problem.