Everyone knows that good ideas come from the shower!
There's something about taking a shower that seems to steam open the pores of creativity. Actually while it isn't the shower itself, there are a number of elements that you can recreate to get to the same outcome:
1. Easy tasks that are repetitive and require no thinking. These tasks allow your mind to wander into more interesting areas - and that seems to help you come up with ideas. You can do the same thing taking a walk, knitting, or practicing scales. Set yourself up to let your mind wander - wash dishes (or your car), take that walk, or get in the shower!
2. Quiet (ok the shower isn't really all that quiet, but it is relatively quiet). Be quiet in quiet - turn off the radio while driving, be outdoors, or find another way to have some quiet to let your mind be quiet - the quiet seems to attract new ideas.
3. Time alone - 'nough said. Even the most lovely people can be a distraction (caveat - sometimes they can also be an inspiration - there are no rigidities here!)
4. No expectations - you don't go into the shower thinking that by the end of the shower you will have developed a completed composition (or solved world hunger). Be fanciful - in effect you can have your own brainstorming session with no idea rejected until a later phase.
So, if you need help to generate good ideas for arrangements for Christmas music, generate a virtual shower: and (this is important) capture the outcomes (so you can build on them). If you are actually in your shower, you can use a grease pencil or water based marker to write on the tile or just keep singing a motif to yourself!