1. Keep outdoor decorations simple and tasteful. If your home looks like Griswold decorated it, buyers may have a tough time actually seeing the home you are trying to sell. Also remember, if your house goes under contract in December, you will have to pack up all those decorations in addition to the contents of your home before moving!
2. Indoor decorations should be simple and tasteful too. If you put up trees, be sure that you are not blocking the normal traffic flow.
3. Holiday scents can help put people in a festive and cheery mood. Just keep them subtle. Remember, there are plenty of people with allergies.
4. Soft music generally is recommended for showings. You might want to shy away from overly religious songs and save those for when the home isn't being shown.
5. Photos marketing your home with holiday decor should be removed no later than the first week of January. I just love seeing "This home won't last long on the market" on homes in MLS in July, with a Christmas tree in the living room photos!
Lastly, it's a nice touch to put out a plate of cookies when you have showing appointments, with a note for buyers to help themselves!