I've had this problem for years, ever since primary school i guess (i'm 25 now). I saw the doctor about it 4 or 5 years ago and he diagnosed it a Benign Essential Tremor - it is basically a relatively harmless nervous condition, in very worst case scenarios it may lead to fairly uncontrollable shaking of hands & head in later life, albeit fairly rare for it to progress that far. There is no cure for the condition but in more serious cases it can be controlled with Beta Blockers as mentioned a few posts up.
You should still go and see your doctor though.