Keep a good attitude! Both fun-loving Thais and tourists are going to splash you with water -- don't get mad or feel singled out. No one is exempt from the wet mayhem.
Be safe. Drunk driving is a serious problem during Songkran; be careful when crossing the road or while standing in the street.
Although the water splashing is fun for everyone, remember that Songkran is actually a religious festival. Stay out of the way of worshipers at temples and shrines.