stick it in microwave for 5 min if it can fit, otherwise put it the oven on 300 degrees for about 2 hours to get rid of any mold or fungus that may be on it naturally. I have found that some natural wood and some store bought drift wood still will mold after these treatments
EDIT:
the only wood product i know of that wont mildew or produce fungus readily is cork bark, I usually do not have to treat it at all before using