The most important thing is too keep your bitcoins away from exchanges before that day and some time after that. You must put it in a wallet where you control the keys. This way, if ever a hard fork happens or not, your bitcoin is safe. If a hard fork happens, then there is a split and you will have equal value of the two resulting coins. Otherwise, business as usual.
That definitely will be the first step. Make sure you are in control of your coins.
The second step is to decide which coin to back. Supporters of both forks will dump the 'other' coin. You wouldn't want to be caught sleeping.