Here's expert advice from a previous BetterPhoto Forum post on how to prevent Christmas lights from overexposing and blowing out: "Your camera meter sees the dark green tree and these little light bulbs, and in trying to determine the best exposure, ends up blowing out, or overexposing the light bulbs. Often, the only way it works is to use supplemental strobes and light boxes. Or use your on-camera flash as a fill light. Put your camera on the tripod and compose your picture. Take some test shots on M for manual, not P or A. Adjust your shutter speed and/or f-stop until the Christmas lights look great. Most likely, the rest of the tree will be very dark, and now you use your flash to open up those areas. Turn on the flash on your camera and shoot another. If it looks too "flashed", then go into the menu and reduce the flash output starting at -1; then if you need more, try -1.3, -1.6, -2. Hopefully, something in there will work."
And, from the same Q&A thread, these exposure thoughts from BetterPhoto member Doug Elliot: I have been working on a project for our local CofC. I am shooting outside lights. I use a tripod and shutter release, and meter for the lights. I shoot in manual mode, tungsten mode for white balance, and Raw. I also bracket my shots.