As long as you don't believe in it you will see that you have as much luck or bad luck as in a normal day. When you believe in it you are more likely to unconsciously do things that you will interpret as bad luck. Don't be scared of it. Just do the things you normally do and don't let your fear effect that.
I actually agree with Oro, I don't think I've ever had a bad Friday the 13th. If you're terribly worried about it, consider getting a leather bracelet or belt and carving some protective sigils and symbols, then charging it and wearing it.
Actually it is a day of luck. Because cultures with a lunisolar calendar have from time to time a year with 13 months (as we have an extra day every four year) the number 13 was considered a symbom of luck.