The block reward is scheduled to be halved on around august 2016.
when that happens unless the amount of transactions fees rises to compensate for the reward drop,
mining will become unprofitable and the hash rate will drop rapidly.
what can be done to prevent this

What needs to be prevented, exactly? And why?
People saying bullshit like "mining will become unprofitable" clearly haven't got a clue how mining and the adaptive difficulty works.
Minnig will always remain profitable for "enough" people. ALWAYS.
That remains true until all Bitcoins are created. Then the only reward is transaction fees which as of right now is pretty much nothing.