Christmas comes but once a year, and picking out the perfect tree to jolly up the living room is a family ritual full of fun and promise. But with all the different types of trees out there, it's hard to know which one is right for your holiday display. And knowing how to keep it green and fragrant once you deck it out is a talent that eludes many a yuletide reveler. "When a tree goes south," says This Old House landscape contractor Roger Cook, "ninety-nine percent of the time it's human error." So before you turn your tannenbaum into the Charlie Brown special, take a few tips from our experts on how to pick one, measure for it, and make sure it lasts through December 25 and maybe into the New Year.