Those tiny glass bulbs that emit light can cause more trouble than all your other Christmas decorations combined. Before trimming your prelit tree, plug the lights in and stand several feet back from the tree. If you see blank spots where lights should be twinkling but aren't, start by tightening the bulbs in these areas. If this doesn't work, you need to replace the bulbs. Most prelit trees and additional strands of lights come with a few such replacement bulbs. If you've used up your supply, they are easy to purchase in almost any store that sells Christmas decorations.