All bitcoins you have on the block chain before the fork, you will have on both forks at the time of the fork. In essence, you will double your bitcoins (sort of). The best way to prevent this is to not have any bitcoins at the time of the fork. One easy way not to have any bitcoins at the time of the fork is to leave "your" bitcoins on an exchange so that the exchange has the bitcoins. That's how most people in the bitcoin space who don't want to have any bitcoins make sure they don't have any bitcoins.