All types of boards, long and short, are prone to speed wobbles. Speed wobbles are the result of Self-Exciting Oscillation; at high speed the energy of your velocity will feed into the natural oscillation of your trucks as they try to return to their natural (neutral) position, continually increasing the amplitude.
There are two ways to reduce this effect:Increase your truck's resistance to oscillation
As mentioned by latetojoin, tightening your trucks and using bushings with a higher durometer will increase resistance to turning. This will increase the critical speed at which the force of your velocity will exceed the point of self-exciting oscillation. This comes at a trade-off of reduced general manoeuvrability, which can be undesirable.Ride relaxed and confident
Unfortunately, this only comes with experience riding at speed. The natural physical response to the loss of control stemming from speed wobbles is to tighten up in an attempt to stay on the...