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 board. This is bad, from a physics system perspective, as it will result in your centre of balance constantly shifting around the board with the oscillation, which will then also feed into the oscillation... becoming a continuous feedback loop. Once you have a feel for what your board is capable of and can ride loose, your body will act as a dampening system for the oscillation, dispersing the energy through micro-movements in your joints rather than oscillation of the trucks. This is why a lot of skaters are unsure how to advise newcomers about overcoming speed wobbles, they just seem to go away with time. Really, this is muscle memory and experience kicking in. Use what you know about skating flat and apply it to skating at speed; keep low, keep centred, relax and commit.