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.
when i say mining will become unprofitable i obviously mean mining will become unprofitable at the current difficulty (or what ever it will be when this event happens).
the reward halving will force both difficulty and network hash rate to drop thus making the network more vulnerable to a 50% attack.
unless of course the transaction fee's will be the vast majority of profits made by miners by then.